1917. Arca occidentalis Philippi. Maury, Bull. Am. Paleontology, vol. 5, No. 29, p. 163, pl. 29, fig. 3. Arca noae of many authors, not of Linnaeus.

Philippi described this species in 1847 as follows: A. testa elongato-oblonga, subquadrangula; latere antico breviusculo, angulo fere recto terminato; postico emarginato; carina obtusa ab apice decurrente; costis regularibus, prominentibus, interstitia longitudinaliter striata subaequantibus; medianis parum minoribus; area dorsali mediocri; ligamento illam maxima ex parte occupante. Long. 22'''; alt. 10'''; crass. 11'''.

Patria: Indiae Occidentales; ad Cubam ligit cl. Pfeiffer.

Type locality: West Indies.

The typical recent specimens are large, transversely elongated shells covered with a heavy epidermis. The outline is conspicuously irregular. The umbones are rather small but acute, compressed, widely separated

and placed well within the anterior third. The dorsal margin is straight and marks the maximum latitude of the shell. From the anterior extremity of the hinge the lateral margin slopes backward to the shallow byssal gape. The posterior portion of the ventral margin is broadly convex, the posterior lateral margin quite strongly concave. The rostrum is well defined from the umbones to the outer margin. The umbonal sulcus is feebly developed and broadens rapidly toward the ventral margin, coinciding at the base with the byssal gape. The radial sculpture is very irregular. As a rule there are 6 to 8 strong well-rounded cords on the anterior portion of the shell, with or without intercalated linear secondaries, and 12 or 15 finer and more closely spaced radials, usually with filamentous secondaries upon the median depression. Between the sulcus and the posterior keel the primaries are low, broad, in many specimens mesially furrowed, about 10 in number, and separated by interspaces of varying width in which from 1 to 5 secondaries are developed. The sculpture between the keel and the dorsal margin is the coarsest upon the shell. The number of primaries is about half a dozen, and they are so disposed that there is quite an area between the outer primary and the dorsal margin, which is either finely striated radially or sculptured only with the incrementals which are best developed upon this portion of the shell. The cardinal area is broad and somewhat kite-shaped, the ligament area more restricted than the cardinal, especially behind the umbones, and scarred with irregular furrows converging toward the umbones. The hinge margin is very finely and sharply serrate, the lateral and ventral margins obscurely crenulated in harmony with the external sculpture. The Oak Grove specimens are young but appear to be identical with the Recent species.

and much more finely sculptured. Arca umbonata Lamarck is more inflated in the umbonal region, is more sharply rostrate, and develops a more imbricated ornamentation.

The occurrence of this tropical species in the Oak Grove is significant. Though present in the Bowden of Jamaica it has not been recognized in Florida in strata older than the Caloosahatchee.

The Recent representatives have a wide geographic distribution, though they are restricted to the shallow waters, usually less than 20 fathoms. The species has been reported from Hatteras to Santa Marta and Cartagena and east to Bermuda. The possession of a byssus by which they can attach themselves to foreign objects gives to this group a very effective

means of dispersion.

Occurrence: Oak Grove sand, localities 2646 (juveniles), 5632 (juveniles). Shoal River formation, localities 10603P, 10608г.

Outside occurrence: Miocene: Moin Hill, Costa Rica; Bowden beds, Jamaica; Gurabo formation, Santo Domingo. Pliocene: Caloosahatchee beds, Florida. Pleistocene: Florida and the West Indies. Recent: Cape Hatteras to Cartagena and east to Bermuda in 1 to 20 fathoms.

Arca umbonata Lamarck

1819. Arca umbonata Lamarck, Histoire naturelle des animaux sans vertèbres, vol. 6, p. 37.

1835. Arca umbonata Lamarck, idem, 2d ed., vol. 6, p. 462.

(Synonymy in part excluded.)

1847. Arca umbonata Lamarck. Philippi, Abbildungen und Beschreibungen Conchylien, vol. 3, p. 14, pl. 4, figs.

1873.

1887.

1887.

1889.

За-с.

Barbatia bonaczyi Gabb, Geology of Santo Domingo,
Am. Philos. Soc. Trans., vol. 15, p. 254.

Arca imbricata Bruguière. Heilprin, Wagner Free Inst.
Sci. Trans., vol. 1, p. 118.

Arca listeri? Philippi. Heilprin, Wagner Free Inst. Sci.
Trans., vol. 1, p. 118.

Arca imbricata Bruguière. Dall, U. S. Nat. Mus. Bull.
37, p. 40.

1917. Arca umbonata Lamarck. Sheldon, Palaeontographica Americana, vol. 1, No. 1, p. 8, pl. 1, figs. 12-17. 1917. Arca umbonata Lamarck. Maury, Bull. Am. Paleontology, vol. 5, No. 29, p. 163, pl. 30, fig. 11.

Lamarck described this species in 1819 as follows: A. testâ transversim oblonga, ventricosâ, angulato-sinuatâ decussatim substriata; umbonibus magnis, arcuatis; latere postico brevissimo.

Type locality: Jamaica. (Recent.)

Arca umbonata is a shell of moderately large size and very irregular outline. The umbones are prominent, widely separated, acute at their summits, feebly prosogyrate, and placed within the anterior third. The hinge line is straight and extends to the anterior extremity of the shell but not to the posterior. The anterior margin between the dorsal area and the byssal opening is smoothly rounded. The opening is rather wide and occupies one-third or more of the margin between the anterior extremity of the hinge posterior keel, which is produced backward beyond the posterior extremity of the hinge. The posterior lateral margin is little or not at all constricted. The surface ornamentation is finely and rather evenly cancellate, the radials standing out more sharply anteriorly, whereas on the posterior area they do not number more than half a dozen and are much coarser than upon the medial or anterior portions of the shell. The cardinal area is wide and smoothly convex, and the ligament occupies the entire area. There are two deep grooves diverging beneath the umbones and forming a small, diamond-shaped area in the double valves. Other shorter grooves to the number of half a dozen are developed near the hinge line. The hinge is very finely serrate within the limits of the pallial line, which follows close to the outer margin. The inner margins are simple.

Arca umbonata is more irregular in outline and more regular in ornamentation than any of the other species of Arca s. s. reported from the Alum Bluff. The umbonal inflation is also higher and the rostral keel sharper than in either Arca occidentalis or Arca paratina.

The Alum Bluff material is very meager and not sufficient to prove without question the identity of the Tertiary and Recent species. The relationship is certainly close, and the apparently wider byssal gape of the Tertiary forms may easily be due to individual variation or to a slight immaturity.

The Recent representatives inhabit the shallow waters along the east coast from Hatteras to southern Brazil and east to Bermuda.

Occurrence: Chipola formation, localities 2212, 2213, 2564, 3419. Oak Grove sand, localities 2646", 5632г.

Outside occurrence: Miocene: "Tampa silex beds," Florida; Moin Hill, Costa Rica, and the Gurabo formation, Dominican Republic. Pleistocene: Florida and the West Indies. Recent: Cape Hatteras to southern Brazil in less than 50 fathoms.

Arca paratina Dall

Plate V, Figure 5

1898. Arca paratina Dall, Wagner Free Inst. Sci. Trans., vol. 3, pt. 4, p. 621, pl. 33, fig. 14.

1917. Arca paratina Dall. Sheldon, Palaeontographica Americana, vol. 1, No. 1, p. 7, pl. 1, figs. 5-7.

Dall describes this species as follows:

Shell elongated, not very thick or high, not much distorted but with a variable byssal gape, inequilateral, the beaks at or near the anterior fourth; moderately alate in front and behind; beaks low, pointed, not inflated, their apices slightly prosogyrate, cardinal area long, narrow, lozenge-shaped, flattish, with longitudinal striae, the site of the resilium marked on each valve by two grooves forming a small triangle, within which are traces of the inception of other grooves; sculpture chiefly of fine radial riblets overrunning and somewhat imbricated by

margin of the byssal foramen are perceptibly finer than the rest, those on the posterior dorsal slope are more or less fasciculated, the ends of the fascicles dentating the posterior margin; on the dorsal anterior part the riblets increase somewhat in size but are not fasciculated; the dorsal border in front is anterior to the rest of the margin; between the dorsal posterior extreme and the ventral posterior angle there is often an irregular but not deep emargination; the borders of the byssal foramen are irregularly emarginate; interior smooth, the margin denticulated by the sculpture except at the foramen; hinge line straight, minutely denticulate; the teeth in the center smaller, those toward the ends inclined outward slightly, above, and a little larger; there are about 23 anterior and 40 posterior teeth, with no marked hiatus between the series. Longitude of shell, 28 millimeters; altitude of hinge line, 8.5 millimeters; of beaks, 10 millimeters; diameter at the umbonal part, 10 millimeters. It is quite possible that the shell grows to a considerably larger size.

This species is distinguishable at once from the A. occidentalis of the same size by its uniformly more delicate and much more numerous ribs and by its greater length in proportion to its height. It is also usually less alate behind and of more uniform, undistorted shape. Differences of form and proportion seem to separate it sufficiently from A. subprotracta Heilprin.

Type: U. S. Nat. Mus. No. 114786. Type locality: No. 2213, 1 mile below Baileys Ferry, Chipola River, Calhoun County, Fla.

The young of Arca umbonata Lamarck are more inflated and more sharply rostrate, and the sculpture is relatively coarser and more evenly fluted. The young of Arca occidentalis are even more similar in general characters but are less delicate and less finely sculptured. Arca yaquensis Maury, so common in the Cercado and Thomonde formations of the Dominican Republic is less elongated transversely.

Occurrence: Chipola formation, localities 10609a, 2213a, 3419P, 2211P.

Subgenus BARBATIA Gray.

1842. Barbatia J. E. Gray. Synopsis of the contents of the British Museum, p. 81.

Type: Arca barbata Linnaeus. (Recent in the Mediterranean.)

Gray describes this subgenus as follows:

The Barbatia are elongated shells, covered with a hairy periostraca; the teeth on the middle of the line are small, of the ends large and oblique.

Dall adds the following remarks: 12

Section PLAGIARCA Conrad

1875. Plagiarca Conrad. Kerr, Rept. Geol. Survey North Carolina, vol. 1, app. A, p. 4.

=Calloarca Dall and authors. (Type: Arca candida Gmelin.)

Not Calloarca Gray, 1857, Annals and Mag. Nat. Hist., 2d ser., vol. 19, p. 369. (Type: Barbatia alternata Reeve.)

quite so finely sculptured as those from Maryland and New Jersey. The radials upon the anterior portion of the shell are quite sharply elevated and granulated and separated by interspaces of approximately their own width. On the medial portion of the shell the primaries are finer and less uniform in size and spacing and toward the posterior portion of the shell are often

(Ripley formation) of North Carolina.)

The monotype of Gray's Calloarca is Arca alternata Reeve, a rare west coast species of the Acar group. As no other species can serve as the type, Calloarca must of necessity become a synonym of Acar. Litharca Gray, 1842, was founded with Byssoarca lithodomus Sowerby as the monotype, a form which is probably nothing more than an Arca candida much distorted by growth in a Lithodomus burrow. In view of the uncertainty and the misfortunes of employing such a type, Litharca may well be rejected in favor of Conrad's clearly defined Plagiarca.

Conrad defines this section as follows:

Shell with a straight hinge margin terminating in an angle, narrow cardinal area, having minute close angulated lines; cardinal teeth very oblique without angles toward the interior margin; short; anterior series with one or two teeth comparatively large, slightly angulated in the middle and very oblique.

Barbatia (Plagiarca) marylandica Conrad

1840. Byssoarca marylandica Conrad, Fossils of the medial Tertiary of the United States, p. 54, pl. 29, fig. 1.

1863. Barbatia (Byssoarca) marylandica Conrad, Acad. Nat. Sci.

Philadelphia Proc. for 1862, p. 580.

1864. Barbatia marylandica Conrad. Meek, Check list of the invertebrate fossils of North America: Smithsonian Misc. Coll. No. 183, p. 6.

1894. Barbatia marylandica Conrad. Whitfield, U. S. Geol. Survey Mon. 24, p. 48, pl. 7, figs. 2-4.

1898. Barbatia (Calloarca) marylandica Conrad. Dall, Wagner Free Inst. Sci. Trans., vol. 3, pt. 4, p. 623.

1898. Barbatia (Calloarca) candida Gmelin. Dall, Wagner Free Inst. Sci. Trans., vol. 3, pt. 4, p. 626 (part). Not Arca candida Gmelin, 1792, and authors.

1898. Barbatia (Calloarca) marylandica Conrad. Dall, U. S.

Nat. Mus. Bull. 90, p. 119, pl. 4, fig. 3. 1904. Arca (Barbatia) marylandica Conrad. Glenn, Maryland Geol. Survey, Miocene, p. 392, pl. 106, fig. 7.

1917. Arca marylandica Conrad. Sheldon, Palaeontographica Americana, vol. 1, No. 1, p. 14, pl. 3, figs. 5-7.

Conrad described this species in 1840, as follows: Shell oblong, compressed, thin, with very numerous radiating granulated striae; beaks not prominent; base much contracted or emarginate anterior to the middle; posterior side dilated, the superior margin very oblique and emarginate; extremity angulated, and situated nearer to the line of the hinge than to that of the base; cardinal teeth minute, except toward the extremities of the cardinal line, where they are comparatively very large and oblique; inner margin entire.

Type: Academy of Natural Sciences of Philadelphia. Type locality: "Cliffs of Calvert," Md. (Calvert formation).

The Chipola individuals seem to be a little higher relatively, a little more regular in outline, and not

intercalated, making a rather crowded ornamentation. Over the keel the radials are very low and irregular, but strengthen again behind the keel, are more sharply corrugated by the incrementals, and are, as a rule, mesially sulcate. The cardinal area is narrow and furrowed with four or five discontinuous, more or less undulatory grooves. The hinge is extremely narrow and finely serrate medially but widens distally, especially behind the umbones. There are about four relatively coarse anterior teeth and half a dozen posterior. The inner margins are simple.

Occurrence: Chipola formation, localities 2212P, 2213P, 2564, 34195, 2211. Oak Grove sand, localities 2646, 7054, 106595. Shoal River formation, locality 10603г.

Outside occurrence: Miocene: "Tampa silex beds" at Ballast Point, Fla.; the marls of Jericho, Cumberland County, N. J., and beds at Plum Point, Calvert Cliffs, and Centerville, Md.

Juveniles referred by Dall to Arca candida Gmelin should be included under this species. The young candida are less finely and more sharply threaded than young marylandica. Arca candida is a denizen of the shallow waters along the east coast from Cape Hatteras to southern Brazil.

Barbatia (Plagiarca) phalacra Dall

Plate V, Figure 6

1898. Barbatia (Calloarca) phalacra Dall, Wagner Free Inst. Sci. Trans., vol. 3, pt. 4, p. 626, pl. 33, fig. 3.

1917. Arca phalacra Dall. Sheldon, Palaeontographica Ameri

cana, vol. 1, No. 1, p. 16, pl. 3, fig. 10.

Dall describes this species as follows:

Shell thin, moderately convex, equivalve, inequilateral; the prosogyrate beaks within the anterior fourth low and somewhat compressed; sculpture of very numerous fine, even, mostly dichotomous riblets without nodules or reticulation over the whole shell, crossed only by feeble incremental lines; cardinal area very narrow with a few longitudinal grooves; hinge teeth small, short, and vertical mesially without any gap in the series, distally longer, larger, and more oblique; hinge line of the whole length; internal margin of the valves smooth, byssal gape inconspicuous. Longitude 23.5, altitude 11, diameter 9 millimeters.

This is a very modest and neat little species which does not seem identifiable with any of the others. It is perhaps nearest to B. mississippiensis Conrad but is smaller, less flattened, and more regular.

Type: U. S. Nat. Mus. No. 135831.

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

this species. The young are very finely and evenly threaded radially. Later on the radials become less uniform in size and spacing and tend to bifurcate. Secondaries are commonly intercalated, and the resulting ornamentation is quite similar to that of the less inflated Barbatia marylandica Conrad. The juvenile uniformity of sculpture persists much longer in the two individuals from Oak Grove than in the Chipola individuals and may, with the acquisition of further material, prove a character of taxonomic value. Occurrence: Chipola formation, locality 2213P. Oak Grove sand, locality 2646.

Section ACAR Gray

1857. Acar J. E. Gray, Annals and Mag. Nat. Hist., 2d ser., vol. 19, p. 369.

Type: Arca gradata Broderip and Sowerby. (Recent off the west coast of Mexico and Central America.)

Shell small, solid, inequilateral, transversely elongated, rectangular, rostrate behind; surface normally cancellate; ligament area rhomboidal and, for the most part, opisthodetic, more restricted anteriorly than the cardinal area.

Arca donaciformis, commonly cited as the type of this section and described by Reeve from Mozambique, is not included in Gray's original list of three species. Arca gradata was designated the type by Woodring.13a

Barbatia (Acar) reticulata Gmelin

1784. Arca reticulata Chemnitz, Conchylien-Cabinet, vol. 7, p. 193, pl. 54, fig. 540.

1792. Arca reticulata Gmelin, Systema naturae, vol. 6, p. 3311. 1819. Arca domingensis Lamarck, Histoire naturelle des

animaux sans vertèbres, vol. 6, p. 40.

1898. Barbatia (Acar) reticulata Gmelin. Dall, Wagner Free Inst. Sci. Trans., vol. 3, pt. 4, p. 629.

1917. Arca reticulata Gmelin. Sheldon, Palaeontographica Americana, vol. 1, No. 1, p. 20, pl. 4, figs. 8-12.

1917. Barbatia (Acar) reticulata Gmelin. Maury, Bull. Am. Paleontology, vol. 5, No. 29, p. 166, pl. 30, fig. 16.

1919. Arca (Acar) reticulata Gmelin. Harris, Bull. Am. Paleontology, vol. 6, No. 31, p. 55, pl. 22, figs. 18, 19. Gmelin described this species in 1792 as follows: A. testa subrhomboidea decussatim striata alba: natibus approximatis, vulva cordata.

and proximate. The external surface is sharply cancellated by conspicuously elevated radial cords separated by deep grooves of approximately their own width and crossed at regular intervals by numerous concentric ridges or laminae which tend to nodulate the radials at their intersection. Between the concentric ridges the interradial grooves appear as deep pits. Along the posterior keel the concentric sculpture is commonly conspicuously laminar or scabrous, and this character is maintained to a certain extent across the posterior area. The cardinal area is extremely narrow, expanding slightly beneath the umbones and scarred with a few irregular, commonly discontinuous grooves rudely parallel to the hinge line. The hinge is rather short and rather coarsely dentate distally, with irregular and in some specimens obsolete medial serrations. The inner margin of the valves is finely dentate.

Two individuals serve to establish the presence of a species unusually well characterized by outline and ornamentation.

Occurrence: Chipola formation, locality 2213г. Shoal River formation, locality 10608г.

Outside occurrence: Eocene: Claiborne group of Mississippi and Texas (?). Miocene: "Tampa silex beds" of Florida; Miocene of Santo Domingo and Jamaica. Pliocene: Caloosahatchee formation of Florida. Pleistocene: Florida and the West Indies. Recent: Cape Hatteras to the Gulf of Campeche and east to Bermuda.

The Recent members have a wide geographic and bathymetric range. Off Hatteras the form occurs both in shallow water and in deep water up to 287 fathoms. Elsewhere in its range along the east coast from Hatteras to the Gulf of Campeche and east to Bermuda it is restricted to the shallow waters. The species has also been reported from the west coast from San Diego to Ecuador, but this is probably a closely allied and not an identical form.

Section FOSSULARCA Cossmann

1887. Cossmann, Catalogue illustré des coquilles fossiles de l'Éocène des environs de Paris, vol. 2, p. 142.

Type: Arca quadrilatera Lamarck. (Eocene of the Paris Basin.)

Cossmann defines this section as follows:

Coquille mince, oblongue, subquadrangulaire, subéquilatérale, inéquivalve, à surface treillisée; crochets écartés; aire ligamentaire étroite, portant sous le crochet une petite fossette triangulaire bien limitée, au fond de laquelle se distinguent de petits sillons perpendiculaires au bord; dents presque égales, plus ou moins obliques.

Shell thin, oblong, subquadrate, subequilateral, inequivalve. Sculpture reticulate. Umbones separated by a narrow cardinal area grooved parallel to the hinge line. Ligament restricted to a minute vertically striated diamond-shaped area directly beneath the beaks. Teeth subequal, more or less oblique.

Barbatia (Fossularca) adamsi (Shuttleworth MS.) Dall

Plate V, Figures 1-4

1845. Arca coelata Conrad, Fossils of the medial Tertiary of the United States, p. 61, pl. 32, fig. 2.

Not Arca coelata Reeve, 1844.

1886. Arca adamsi Shuttleworth. Dall, Harvard Coll. Mus. Comp. Zoology Bull., vol. 12, p. 243.

1888. Arca (Acar) adamsii Shuttleworth MS.? Smith, E. A., Linnaean Soc. Jour. Zoology, vol. 20, p. 499, pl. 30, figs. 6, 6a.

1898. Barbatia (Fossularca) adamsi (Shuttleworth) Smith. Dall, Wagner Free Inst. Sci. Trans., vol. 3, pt. 4, p. 629. 1917. Arca adamsi (Shuttleworth) Smith. Sheldon, Palaeontographica Americana, vol. 1, No. 1, p. 22, pl. 4, figs. 16-18, pl. 5, fig. 1.

Conrad described this form in 1845 as follows:

Trapezoidal, disk widely and not profoundly contracted; ribs numerous, alternated toward the base, tuberculated, aculeated anteriorly and posteriorly; posterior slope depressed; umbo acutely angulated behind; basal margin slightly arched; posterior margin obliquely truncated; beaks approximate. Locality, Wilmington, N. C.

Dall described this species in 1886 as follows:

This species is very common in shallow water throughout the West Indies and extends northward nearly or quite to Cape Hatteras. Its simulated ribs of trailing blisters give it a remarkably similar appearance to Arca lactea, which, however, has real ribs. There is a dwarf, very short squarish variety, which from its greater proportional diameter (though not otherwise different) would at first be separated as distinct and which may be called Arca adamsi var. conradiana.

Figured specimen: U. S. Nat. Mus. No. 325489 (Wilmington, N. C.).

Type locality: West Indies.

Though this species was named by Shuttleworth in the early fifties, and though it is widely distributed, no record of any published description or figure has yet been found. The name appears in several check lists by Beau, Krebs, and others, but Dall seems to have been the first to give any criteria for its identification. The formal Latin description of Smith was not published until two years later.

The shell is unusually distinctive and may be readily recognized by the trapezoidal outline, the contraction of the basal margin, the dainty cancellated sculpture with its riblets formed of hollow flutings, the transverse ligamental grooves confined within a small diamond-shaped area directly beneath the umbones, and the well-defined muscular impressions mounted on raised ridges which converge dorsally beneath umbones. The measurements given by Smith are: Longitude, 12 millimeters; altitude, 7.5 millimeters; diameter, 7.5 millimeters.

There are no characters by which the Alum Bluff forms may be separated from the Recent. In most of the individuals, however, the sculpture is more or less decorticated, leaving a roughened limy surface. The Recent members occur in considerable abundance from Hatteras to Fernando de Noronha, Brazil, in waters 35 fathoms in greatest depth. The species has also been reported from Bermuda.

Occurrence: Chipola formation, localities 2213°, 9994, 7468. Oak Grove sand, localities 2646, 5632. Shoal River formation, localities 9957, 5618г.

Outside occurrence: Miocene: Bowden formation, Jamaica; Yorktown formation, Virginia; Duplin formation, North Carolina, South Carolina, Georgia. Pliocene: Waccamaw formation, South Carolina; Caloosahatchee formation, Florida; Croatan formation, North Carolina. Pleistocene: South Carolina, Florida. Recent: Cape Hatteras to Fernando de Noronha, Brazil, and east to Bermuda in 10 to 35 fathoms.

Subgenus DILUVARCA Woodring

1925. Woodring, W. P., Miocene mollusks from Bowden, Jamaica: Carnegie Inst. Washington Pub. 366, p. 40= Scapharca of authors, not of Gray, Zool. Soc. London Proc., p. 198, 1847.

Type: Arca diluvii Lamarck. Miocene to Recent, Mediterranean Sea.

The following is a description of Diluvarca:

Shell heavy, medium-sized, moderately elongate, strongly inflated, inequilateral, equivalve, valves closed along ventral margin; umbos high and full; sculpture consisting of strong, narrow, flattened radial ribs separated by squarely channeled interspaces, ribs beaded by concentric threads; cardinal area relatively wide, on adult shells almost entire area occupied by ligament and bearing chevron-shaped ligament grooves diverging from under umbo at an obtuse angle; hinge uninterrupted but consisting of two series of teeth unequal in length; the shorter anterior series comprising teeth that are slightly oblique at anterior end and become vertical but not much reduced in size at posterior end; the longer posterior series comprising teeth that are strongly oblique at posterior end and become vertical and very small at anterior end; posterior muscle scar quadrangular, larger than the rounded anterior scar; margin of valve deeply fluted.

Diluvarca (Diluvarca) latidentata (Dall)

Plate V, Figure 7

1898. Scapharca (Scapharca) latidentata Dall, Wagner Free Inst. Sci. Trans., vol. 3, pt. 4, p. 638, pl. 32, fig. 15. 1915. Scapharca latidentata Dall, U. S. Nat. Mus. Bull. 90, p.

121, pl. 25, fig. 2 (part).

1917. Arca latidentata Dall. Sheldon, Palaeontographica Americana, vol. 1, No. 1, p. 33, pl. 7, figs. 17-20.

Dall described this species in 1898 as follows: Shell small, ovate, moderately convex, with low, quite anterior, mesially sulcate, prosocoelous beaks; left valve with about 30 rounded, radiating, undivided ribs, separated by slightly wider interspaces and crossed by numerous smaller concentric ridges which become beadlike on the ribs and vary in prominence in different specimens; base evenly arcuate, ends rounded; cardinal area narrow, impressed, smooth, with one or two grooves behind the beaks but none elsewhere; valves slightly twisted, so that the basal margin is not in a single plane; line of teeth interrupted a little behind the beaks, the anterior series having the anterior and posterior teeth larger and the intervening teeth thinner and more closely