adjacent, all nearly vertical; posterior teeth vertical, shorter, the series longer, the teeth smallest proximally and regularly increasing in size toward the distal end of the series, equidistant and regular; inner margin of the valve deeply fluted. Longitude 18, height 11, diameter 9 millimeters.

This little shell looks a good deal like the young of Anadara aresta Dall but has the beaks less central, less prominent, and distinctly impressed mesially, giving a somewhat bilobed aspect to the very young.

Type: U. S. Nat. Mus. No. 107720.

Type locality: No. 2564, McClelland farm, 1 mile below Baileys Ferry, Calhoun County, Fla.

There is no Diluvarca reported froin the Alum Bluff which shares with latidentata the evenly arcuate base and the narrow, well-elevated radials separated from one another by interspaces of approximately their own width.

Occurrence: Chipola formation, localities 106095, 7893, 2212P, 22145, 2213°, 2564", 3419г.

Diluvarca (Diluvarca) mikkula Gardner, n. sp.

Plate V, Figures 8-9

Shell very small, slightly inequivalved, transversely elongated, subrectangular. Umbones rather full, moderately elevated, the apices turned inward and slightly forward and placed at some little distance in front of the median vertical. Length of dorsal margin approximating but not equaling the maximum latitude of the shell. Anterior margin intercepting the dorsal at nearly a right angle, expanding slightly in front of the dorsal margin. Base line broadly and feebly constricted at the terminus of the shallow umbonal depression, expanding posteriorly. Posteroventral margin slightly produced and rounded. Posterior lateral margin oblique, intercepting the dorsal margin at an acute angle. Umbonal depression shallow but distinct. Posterior keel obscurely defined toward the umbones, obsolete away from the umbones. Radial sculpture of low, narrow bands widening laterally, especially upon the posterior slope, 23 in number, separated by angular, interradial channels of approximately their own width. Concentric sculpture restricted to exceedingly fine laminae, visible for the most part only in the interspaces on the posterior half of the shell but in some specimens overriding the radials from the umbones to the postero-ventral margin. Margins crenate in harmony with the radial sculpture. Cardinal area narrow, produced posteriorly, expanding somewhat beneath the umbones. Area of ligament attachment rhomboidal, coincident anteriorly with the cardinal area but more restricted posteriorly; one or two grooves directed obliquely backward from the umbones, the more shallow of the two defining the posterior limit of the ligament. Inner margin of hinge broadly and feebly arcuate. Hinge teeth very fine, especially beneath the umbones, less fine and more obliquely distal. Adductor scars 51459°-26-3

obscure, placed well up toward the hinge. Anterior scar roundish, smaller and more regular in outline than the posterior.

Dimensions: Altitude, 3.5 millimeters; latitude, 6.0 millimeters; diameter, 3.2 millimeters. Type: U. S. Nat. Mus. No. 352305.

Type locality: No. 2211, Alum Bluff (lower bed), Liberty County, Fla.

This small and prolific bivalve has been considered to be the young of Diluvarca hypomela (Dall), but it is much heavier than hypomela of the same size and more nearly rectangular. The umbonal depression is less pronounced, and the radials are broader and flatter and normally less than 30 in number, whereas those of hypomela commonly exceed 40.

Occurrence: Chipola formation, localities 2213a, 2564. 3419a, 2211a, 7183°, 10660г.

Diluvarca (Diluvarca) hypomela (Dall)

Plate V, Figure 12

1898. Scapharca (Scapharca) hypomela Dall, Wagner Free Inst. Sci. Trans., vol. 3, pt. 4, p. 637, pl. 33, fig. 1. 1913. Scapharca hypomela Dall, U. S. Nat. Mus. Bull. 90,

p. 121, pl. 17, fig. 7, pl. 21, figs. 17, 18.

1917. Arca hypomela Dall. Sheldon, Palaeontographica Americana, vol. 1, No. 1, p. 34, pl. 7, figs. 22-25.

Dall described this species as follows:

Shell of moderate size, long, inflated, with rather low, mesially compressed, prosogyrate beaks; left valve with about 43 deeply channeled, flat-topped ribs with fine, regular, concentric beading, except on the posterior slope, where the ribs are lower, flatter, and obsoletely channeled; near the margin some of the ribs have a second set of finer grooves; hinge line straight, anterior end descending vertically, then obliquely rounded into the base, which is nearly parallel with the hinge line; the posterior end descends more obliquely and the basal angle is prolonged a little and rounded; the interspaces between the ribs in both valves are very narrow, and on the right valve the beading is less conspicuous; the cardinal area is somewhat concave, flattish, with three or four concentric grooves in lozenge form; teeth of the hinge similar, numerous, not interrupted, short, vertical, the distal teeth a little longer and more oblique; margin of the valves fluted, the right valve slightly smaller than the other. Longitude 50, altitude 25, diameter 20 millimeters.

This species has the appearance of being the Oligocene ancestor of the Miocene A. lienosa, from which it differs by its smaller size, closer and rather narrower ribbing.

Type: U. S. Nat. Mus. No. 114800. Type locality: No. 2212, Tenmile Creek, 1 mile west of Baileys Ferry, Calhoun County, Fla.

The Ballast Point individuals are smaller, heavier, and more inflated than those from the Chipola and present differences of at least subspecific value. The casts from Sopchoppy indicate a more evenly inflated shell and an earlier and more constant bifurcation of the radials. The rather ornate sculpture of the adult hypomela together with its transversely elongated outline is sufficient to separate it from the coexistent members of the genus. The Oak Grove Diluvarca dodona is higher relatively and has higher, more inflated umbones, a less obtuse rostrum, and more finely divided radials. Arca henikeni Maury from the Cercado and Gurabo formations of the Dominican Republic is intermediate in outline between Diluvarca hypomela Dall and D. dodona from the Oak Grove. The details of the sculpture, however, are more simple than in either of the Alum Bluff species. Diluvarca acompsa from the Chipola suggests an adolescent hypomela but is more uniform in altitude in the anterior and posterior portions of the shell, the base line running approximately parallel to the dorsal margin throughout its extent. The very young hypomela may be separated from D. mikkula by their thinner shells, more irregular, somewhat bilobate outline, and more numerous radials.

Occurrence: Chipola formation, localities 10609P, 7893, 2212P, 7257, 2213°, 2564P, 3419°, ?7468г. Oak Grove formation, locality 56305. Shoal River formation, locality 10603P.

Diluvarca (Diluvarca) acompsa (Dall)

Plate V, Figure 11

1898. Scapharca (Scapharca) acompsa Dall, Wagner Free Inst. Sci. Trans., vol. 3, pt. 4, p. 648, pl. 33, fig. 15. 1917. Arca acompsa Dall. Sheldon, Palaeontographica Americana, vol. 1, No. 1, p. 34, pl. 7, fig. 21.

Dall described this species in 1898 as follows:

Shell rectangular, elongate, rather compressed, with low prosocoelous beaks, situated at about the anterior fourth of the whole length; right valve with about 36 flattened radial ribs, with much narrower interspaces; the anterior (22) ribs are mesially divided by a sharp groove and feebly rippled above; the posterior ribs are flat, smooth, and increase in width backward; the anterior end of the shell is evenly rounded, the base straight and parallel with the hinge line, the posterior end wider, a little produced below and with a conspicuous angle above; cardinal area long, very narrow, with one or two grooves, and bordered behind with an elevated margin; hinge line straight, long, with numerous small, uninterrupted teeth, very short mesially, longer and somewhat more oblique distally; inner margin of the valves fluted, shell thin and delicate. Longitude 20, altitude 10.5, semidiameter 4.5 millimeters.

Only two right valves of this little species have been examined. It resembles the young of A. hypomela but is immediately distinguishable by its more compressed and rectangular form and smooth, flat posterior ribs.

Type: U. S. Nat. Mus. No. 114794. Type locality: No. 2212, Tenmile Creek, 1 mile west of Baileys Ferry, Calhoun County, Fla.

In the young hypomela, especially those too young to have developed the sculptural details, the posterior portion of the shell is decidedly broader than the anterior, so that the base line is not parallel to the hinge as in acompsa but oblique to it.

No further valves referable to this species have been found.

Occurrence: Chipola formation, localities 2212",

2213г.

Diluvarca (Diluvarca) dodona (Dall)

Plate V, Figures 14-15a

1898. Scapharca (Scapharca) dodona Dall, Wagner Free Inst. Sci. Trans., vol. 3, pt. 4, p. 640, pl. 31, figs. 1, 8, 8a. 1917. Arca dodona Dall. Sheldon, Palaeontographica Americana, vol. 1, No. 1, p. 37, pl. 8, figs. 6-10.

Dall described this species in 1898 as follows: Shell small, solid, inequilateral, inflated, and rounded in front, pointed and attenuated behind; with mesially impressed, prosocoelous beaks; left valve with 36 squarish radial ribs, each with a deep central groove longitudinally, the portions on each side with a shallower longitudinal sulcus, so that each rib, except in young shells, is composed of four threads set in two pairs; the ribs separated from each other by channeled interspaces about half as wide as the ribs; concentric sculpture of numerous rather close set, regular, blunt, elevated lines, which appear on the riblets as fine undulations; beaks at the anterior third; cardinal area, with a raised margin, lozenge-shaped, rather wide, slightly narrower behind the beaks, with about four rather wavy sets of concentric grooves; hinge line short, solid, the teeth not interrupted, larger distally, the most anterior tending to break up into granulations, about 50 in all, subvertical, shorter in the middle of the hinge; margins of the valve deeply fluted; right valve with wider interspaces and narrower, often tripartite, ribs. Longitude 40, altitude 28, diameter 30 millimeters.

This fine shell has a neat and elegant surface sculpture, and is one of several which the Oak Grove marl contains and which appear to be new.

Type: U. S. Nat. Mus. No. 107385.

Type locality: No. 2646, Oak Grove, Yellow River, Okaloosa County, Fla.

Diluvarca dodona is the most ornate of all the Diluvarcas reported from the Alum Bluff. Diluvarca hypomela from the Chipola presents the same general type of sculpture but is relatively lower and more elongated transversely and the ribs are more numerous and cut by only a single longitudinal sulcus.

The species is prolific and widely distributed throughout the Oak Grove.

Occurrence: Oak Grove sand, localities 2646 pr, 5632°, 5631P, 5630P, 5633P, 7054°.

Diluvarca (Diluvarca) staminata (Dall)

Plate V, Figure 13

1898. Scapharca (Scapharca) staminata Dall, Wagner Free Inst. Sci. Trans., vol. 3, pt. 4, p. 641, pl. 31, fig. 11 (fig. 13 excluded).

1917. Arca staminata Dall. Sheldon, Palaeontographica Americana, vol. 1, No. 1, p. 39 (part), pl. 9, figs. 4, 5 (fig. 6 excluded).

Dall described this species in 1898 as follows: Shell of moderate size, plump, rhombic, with well-elevated hardly sulcate, slightly prosocoelous beaks, situated in the anterior third of the length; left valve with 28 or 29 radial ribs, the posterior of which are smooth and almost rounded; those on the middle of the valve are squarish, with wider channeled interspaces, and rippled or furnished with transverse nodulation above, which grows stronger and more crowded anteriorly; the ribs are not sulcate or dichotomous, and hardly differ on the two valves; hinge line straight, rather long, and with conspicuous angles at the ends; anterior end of the valve rounded, base nearly parallel with the hinge line, posterior end somewhat produced; beaks narrow, cardinal area with from three to five sets of lozenge-shaped groovings; hinge strong, the teeth in two adjacent series, somewhat oblique, smaller mesially, at the anterior end of the hinge sometimes more or less broken into granules; inner margin of the valves fluted, interior radially striate. Longitude of a large valve 47, altitude 37 millimeters, longitude of figured shell 39, altitude 30, diameter 28 milli

meters.

This species differs from A. santarosana, which occurs in the same beds, by its more rhombic form, proportionately longer hinge line, and unsulcate ribs. It is also a larger and less elegantly sculptured shell. A. staminea Say, of which staminata may prove to be an Oligocene race, has a proportionately longer hinge line, is more sharply truncate behind and more obliquely rounded in front, the beaks are less elevated and wider, the ribs anteriorly are only sparsely and feebly nodular, while the aspect of the whole shell is less elegant.

Type: U. S. Nat. Mus. No. 149025.

Type locality: No. 2211, Alum Bluff (lower bed), Liberty County, Fla.

Diluvarca staminata is another of the surprisingly numerous species peculiar to the coarse sands of Alum Bluff. Though contemporaneous forms, Diluvarca staminata and Diluvarca santarosana geraetera seem to be mutually exclusive, as the one is known only from Alum Bluff, the other from the Chipola. The specimens from Escambia County, Ala., tentatively referred to D. staminata are more closely allied to D. santarosana geraetera. Diluvarca chiriquiensis (Gabb) from the Thomonde formation of Haiti differs from all of the Alum Bluff members of the staminata group in the equal valves, the more obtuse posterior keel, the much broader, more closely scored cardinal area, and consequently more widely separated umbones.

By a curious error an individual of a quite distinct species-Diluvarca waltonia Gardner-from Walton County was figured by Dall.

Occurrence: Chipola formation, localities 2211o, 7183°, 10660°, ? 7468P.

Diluvarca (Diluvarca) santarosana (Dall)

Plate VI, Figures 1-2

1898. Scapharca (Scapharca) santarosana Dall, Wagner Free Inst. Sci. Trans., vol. 3, pt. 4, p. 641, pl. 31, figs. 2, 10. 1917. Arca santarosana Dall. Sheldon, Palaeontographica Americana, vol. 1, No. 1, p. 38, pl. 9, figs. 1-3. Dall described this species in 1898 as follows:

Shell small, short, plump, rostrate, with moderately elevated, mesially sulcate prosocoelous beaks; left valve with 30 elevated, squarish, radial ribs, separated by slightly narrower channeled interspaces; the ribs on the posterior slope are low, smaller, and nearly smooth; those on the middle of the shell have mostly near the margin a shallow mesial sulcus; in those still more anterior the sulcus is deeper and wider, dividing each rib over most of its length into two more or less rounded riblets; concentric sculpture of regularly spaced elevated lines, which on the ribs appear as prominent ripples; right valve having the ribs narrower and less strongly sculptured, and the sulci less distinct; cardinal area short, with about three concentric grooves; beaks within the anterior fourth; hinge line short, with about 57 rather irregular, closely adjacent, nearly

vertical teeth, longer and more oblique distally; margins strongly fluted; base flexuous, posterior end narrow, pointed, without any marked angle at the end of the hinge line. Longitude 36.5, altitude 28, diameter 28 millimeters.

This species is most nearly related to A. staminata Dall, from which it can be distinguished especially by its lower beaks, more oblique posterior slope, more flexuous base, and attenuated posterior end.

Type: U. S. Nat. Mus. No. 107386. Type locality: No. 2646, Oak Grove, Yellow River, Okaloosa County (formerly Santa Rosa County), Fla.

The Chipola specimens have been segregated under the subspecies geraetera because of their relatively higher altitude consequent upon the more elevated and narrower umbones. The two forms occupy about the same relative position in the faunas which they characterize.

The species that occurs abundantly in the vicinity of Bainbridge, Decatur County, Ga., is for the most part the typical santarosana, though there is a tendency toward variation along the lines of geraetera. Apparently a second species or subspecies is present at this locality, though represented only by immature forms. These individuals differ from the young santarosana in their much lower, broader outline. The ornamentation and cardinal area, however, offer no distinguishing characters, and the forms in question are left nameless for the time being.

Occurrence: Oak Grove sand, localities 3386°, 3385°, 7148°, 3396°, 2646a, 5632P, 56315, 5630P, 5633′, 7054P, 9961, 7055P, 10659a. Shoal River formation, locality ?51925.

Diluvarca (Diluvarca) santarosana geraetera Gardner, n. subsp.

Plate VII, Figures 1-2

1917. "Arca staminata" Sheldon, Palaeontographice Americana, vol. 1, No. 1, pl. 9, fig. 6. (Figs. 4 and 5 excluded.) Not Scapharca (Scapharca) staminata Dall, 1898.

Shell of moderate dimensions, heavy, the left valve a little larger than the right. Trigonal in the exterior view, the interior, excluding the cardinal area and the prominent umbones, subrhomboidal. Umbones rather narrow but full, conspicuously elevated, incurved and prosogyrate, the apices within the anterior third. Dorsal margin straight, a little shorter than the greatest width of the shell. Anterior margin slightly constricted at the hinge, broadly and feebly rounded, merging smoothly into the base line, which is somewhat compressed in the adults directly in front of the posterior keel. Rostrum subacute and well defined from the umbones to the ventral margin. Posterior area obliquely flattened, the posterior margin intercepting the dorsal at an angle of more than 90°. Ribs of right valve normally 31 or 32, including the 10 or 11 upon the posterior slope; costals on anterior part of shell corrugated by the incrementals and mesially sulcate, the groove being deepest, the wrinkles heaviest, and the spacing widest along the curve from the anterior to the medial portion of the shell; depression in front of posterior keel emphasized by the disappearance of all ornamentation upon the costae; posterior ribs relatively narrow, rounded, and feebly corrugated. One more rib in left valve than in right; ribs of left valve corrugated or nodose and feebly sulcate upon the antero-medial arch, narrower and less closely spaced posteriorly than upon the medial portion of the shell. Inner margins fluted in harmony with the external sculpture. Cardinal area broad, rudely kite-shaped in the double valves, scored with sinuous ligament grooves, which are rudely concentric and tend to arch up toward the umbones. Hinge very narrow, feebly arcuate along its inner margin, finely serrate; the teeth not far from 50, arranged in two series and tending to break up and become irregular at the extremities of each series. Muscle impressions rather large, well defined, the posterior larger, less regular, and more remote from the hinge than the anterior and, in the adult, feebly buttressed. Pallial line running parallel and rather close to the outer margins.

Dimensions: Altitude of right valve, 35.0 millimeters; left valve, 36.5 millimeters. Latitude of right valve, 37.5 millimeters; left valve, 39.0 millimeters; diameter, 37.0 millimeters.

Cotypes: U. S. Nat. Mus. No. 352308. Type locality: No. 2212, Tenmile Creek, 1 mile west of Baileys Ferry, Calhoun County, Fla.

The subspecies geraetera differs from santarosana s. s. in the narrower and more elevated umbones. This dimensional difference is so constant in the great majority of the specimens from Chipola River and appears so early in the shell that it is worthy to be recognized in the nomenclature. There are a number of young forms apparently unrepresented in the adult which differ from the normal geraetera in the more produced and angulated posterior keel. In the normal young of geraetera the base line is approximately parallel to the dorsal margin, whereas in the adult they converge slightly posteriorly. In the abnormal young forms mentioned, however, the base line is pulled down at the keel, so that it diverges from the dorsal margin and simulates in external aspect some of the coexistent Cardium (Fragum). In the absence of adults these juveniles are included under geraetera.

Occurrence: Chipola formation, localities 10609°, 7893P, 2212°, 7257P, 2213a, 2564a, 3419°, 9994p, 7151°, 3704P, ?7468P, 5613°, 5629a.

Diluvarca (Diluvarca) waltonia Gardner, n. sp.

Plate VI, Figures 3-6

1898. Scapharca (Scapharca) staminata Dall, Wagner Free Inst. Sci. Trans., vol. 3, pt. 4, pl. 31, fig. 13 (description excluded).

1898. Scapharca (Scapharca) staminea Dall, Wagner Free Inst. Sci. Trans., vol. 3, pt. 4, p. 642 (part).

Not Arca staminea Say, American marine conchology, pl. 36, fig. 2 and text, 1832.

1898. Scapharca (Anadara) clisea Dall, Wagner Free Inst. Sci. Trans., vol. 3, pt. 4, p. 657 (part).

Shell rather large, heavy; the left valve a little the larger of the two, ovate-trigonal in outline when viewed from the exterior. Umbones broad and well rounded, elevated high above the hinge line, incurved and feebly prosogyrate, set a little in advance of the median vertical. Anterior area rounding smoothly into the medial. Posterior area obliquely flattened and delimited by a subacute keel which persists from the umbones to the outer margin. Dorsal margin approximately the greatest width of the shell but a little less produced posteriorly. Anterior lateral margin approximately at right angles to the dorsal for a short distance then merging smoothly into the upcurved base. Ventral margin nearly parallel to the dorsal, though inclined to diverge slightly behind. Postero-ventral margin produced beyond the dorsal, the posterior lateral margin consequently oblique. Right and left valve discrepantly sculptured, each, however, with approximately 30 ribs; anterior costals of right valve corrugated or obtusely nodulated by the modified incremental sculpture, the costal ornamentation becoming reduced on the medial portion of the shell and nearly obsolete on the half dozen ribs directly in front of the keel; the half dozen ribs directly behind the keel also simple and more rounded than on any other portion of the shell; the four or five radials nearest the posterior margin relatively very narrow and finely nodulated; all the costals upon the left valve nodose in the young forms, those upon the posterior area, however, becoming simple in the adults; interradials relatively wider and more angular in the right valve than in the left. Inner margin sharply fluted in harmony with the external sculpture. Cardinal area wide in the adults, rising quite steeply from the hinge to the umbones, little or not at all concave, rhomboidal in the double valves, scored with irregular grooves, commonly discontinuous but arranged roughly in concentric rhomboids, about eight in the adult. Hinge narrow but rather heavy, reinforced distally, very finely and closely serrate. Teeth arranged in two series which meet with a barely perceptible interruption directly behind the umbones; anterior series including about 20 denticles, the posterior from 25 to 30; teeth in the medial portion of the hinge normal to the margin, those near the distal extremities more oblique and irregular. Adductor scars very large and distinct, the anterior more regular in outline and placed nearer to the dorsal margin than the posterior. Pallial line distinct, running parallel and quite close to the ventral margin.

Dimensions: Altitude, 32.5 millimeters; latitude, 33.0 millimeters; diameter, 36.0 millimeters.

Type: U. S. Nat. Mus. No. 352306; figured specimen, U. S. Nat. Mus. No. 113827.

Mossyhead, Walton County, Fla.

Diluvarca waltonia is the close analog of Diluvarca santarosana from the Oak Grove and its Chipola subspecies geraetera. It is the heaviest of the three forms and relatively the highest. The umbones are more tumid than those in either of the older forms, the depression in front of the posterior keel is obscure or obsolete, and the base line is consequently straighter. The cardinal area is higher as a rule than that of the Oak Grove species, and the hinge is less delicate. The only differences in sculpture are those concomitant with the thicker, ruder shell of waltonia. Though the adults of these forms can be readily separated, many of the immature individuals are indistinguishable.

Diluvarca clisea (Dall), to which the specimens from Walton County have occasionally been referred, is a larger, more oblique form with the postero-ventral extremity of the shell pulled downward and backward. The two valves of clisea are similarly sculptured, and the number of ribs in some specimens runs as low as 26. Diluvarca staminata is a broader species with a higher, more concave, and more nearly symmetrical cardinal area. Diluvarca staminea Say is much closer to D. waltonia than either clisea or staminata and its possible descendant, a hypothesis strengthened by the closer similarity of the Shoal River forms to the specimens from the Calvert than to those from the St. Marys. The Maryland species offers a wide range of variation in outline, but those individuals in which the relative altitude is as great as in waltonia develop a higher cardinal area. The average number of ribs is lower in the Maryland form, and those directly in front of the posterior keel are normally bisulcate or trisulcate, a character not observed in any waltonia.

Occurrence: Shoal River formation, localities 3856pr, 3732P, 3742a, 3731°, 10658P, 5080, 5184P, 5195", ?5079*, 10661°, 51935, 51945, 3733P, 2238, 9958P, 3748°, 3747, 7264, 9960P, ?10603°, 5618°, ?9959°.

Diluvarca (Diluvarca) strebla Gardner, n. sp.

Plate VIII, Figures 1-2

Shell of moderate dimensions, rather thick, the left valve a little the larger, transversely elongated, broadly and quite strongly inflated from the umbones to the ventral margin, somewhat constricted toward the posterior extremity, especially in the right valve, thus giving to the shell a slightly warped appearance. Umbones broad, very full, incurved and prosogyrate, the apices compressed and feebly sulcated medially, placed a little in front of the median vertical and rather close together. Posterior keel obsolete, the posterior slope of the umbones more sharply rounded, however, than the anterior. Dorsal margin straight, a little more than three-fourths as long as the maximum

from the dorsal, which it intercepts approximately at right angles, and merging into the upcurved base. Ventral margin expanded anteriorly so that the greatest altitude is in plumb with the umbones, constricted posteriorly in the adults but not in the young. Posterior extremity in the adults narrow and quite sharply rounded, the lateral margin intercepting the dorsal at an angle not far from 135°. Sculpture very similar in the two valves, the costals of the left valve being a little more elevated and more wrinkled by the concentric sculpture than those of the right. Radials simple, the same in number in the two valves-approximately 34-narrower and more closely spaced medially and less modified by the incremental sculpture than toward the extremities. Concentric lamination very fine, close, and even, corrugating the anterior costals, especially in the younger forms, but on the posterior portion of the shell almost obsolete, except in the channels. Interradials a little narrower as a rule than the radials, though commonly a trifle wider upon the medial ventral portion of the adult left valve. Inner margins fluted in harmony with the external sculpture. Cardinal area rather narrow, concave, rhomboidal, the portion behind the umbones more produced and attenuated than that in front of it. Ligament grooves in the double valves arranged in about half a dozen more or less regular, concentric rhomboids. Hinge narrow but rather heavy, reinforced distally, very finely and closely serrate. Teeth arranged in two series, their interior extremities adjoining one another with a barely perceptible break, the number in the anterior series running as high as 35, in the posterior series up to 45; serrations in the medial portion of the hinge very fine, close, and at right angles to the margin, coarser and somewhat oblique toward the outer extremities. Adductor scars large and prominent, the anterior a little more rounded, not quite so large and placed a little higher than the posterior, which is rudely quadrate in outline and elongated parallel to the posterior margin of the cardinal area. Pallial line distinct, parallel to the outer margin and not far distant from it.

Dimensions: Altitude, 27.0 millimeters; latitude, 41.0 millimeters; diameter, 26.0 millimeters. Type: U. S. Nat. Mus. No. 352309.

Type locality: No. 3856, 6 miles west-northwest of Mossyhead, Walton County, Fla.

No similar species has been collected from the Alum Bluff group. The upper bed at Alum Bluff, the Choctawhatchee marl, contains a possible descendant"Scapharca (Anadara)" aresta Dall. The younger form is smaller, a little lower relatively, and less tumid, but it exhibits the same peculiarities of outline and the same type of sculpture. The closest affinities of strebla, however, are with a probably synchronous species of the Diluvarca section, Diluvarca actinophora