82 EOCENE MOLLUSCA. 



4. Arca depressa, J. Sowerby. Tab. XIV, fig 4, a — c. 



Arc a depressa. J. Sow. Min. Conch., t. 474, fig. 2, 1824. 

 — — Morris. Catal. Brit. Foss., p. 185, 1854. 



Spec. Char. A. testa tenui, elongato-oblongd, valde incequitaterali, depressa, radiatim 

 striata et concentrice decussatd ; striis tenuibus, distantibus, subtuberculatis ; marginibus 

 ventrali et dorsali subparallelis. 



Shell thin, elongately oblong, very inequilateral, depressed, radiately striated, and con- 

 centrically decussated ; striae thin, and slightly tuberculated ; ventral and dorsal margins 

 nearly parallel. 



Length, phs of an inch ; height, \ an inch. 



Localities. Woolwich {Soiverby), Basingstoke (Prestwich). 



The specimens of this species that I have seen are very few and imperfect, and the 

 characters for specific determination are not satisfactorily displayed. The principal distinc- 

 tion appears to be its depressed form, or shallow valve ; the striae upon the exterior Mr. 

 Sowerby describes as " very distant upon the anterior side (siphonal region ?), and appear 

 like small knotted threads ;" those on the shorter or pedal region are close, with inter- 

 spaces of the same width. The cast of a species of Arca in Mr. Prestwich's cabinet from 

 New Cross, Tab. XV, fig. 15, has the above name (in MS.) attached, and I have had it 

 figured, as it appears somewhat to differ in being more inequilateral; but it is scarcely 

 possible to determine a species from a cast alone. 



5. Arca Dulwichiensis, Edwards, MS. PL XV, fig. 6, a, b. 



Spec. Char. A. testa ovato-oblongd, subobliqud, gibbosuld, valde incequilaterali, 

 utrdque extremitate late obtusd, in medio depresso-sinuosd, radiatim costellatd ; costulis 

 angustis prominulis, in siphoni-rcgione distantioribus ; cardine — ? 



Shell ovately oblong, slightly oblique, somewhat gibbous, greatly inequilateral ; each 

 extremity broadly obtuse ; ventral margin slightly sinuated, covered with radiating riblets, 

 rather distant upon the siphonal region ; hinge — ? 



Length, \\ inch ; height, ^ths of an inch. 



Locality. Dulwich {Edwards). 



A few specimens of a shell, apparently belonging to this genus, have been turned out 

 of the " diggings " for the main sewer at Dulwich ; they enrich the cabinet of Mr. 

 Edwards. These specimens, unfortunately, do not exhibit to view the hinge-area; but 

 the exterior has the form and sculpture which generally characterise the genus Arca. The 

 valves have been pushed a little out of their natural position, and display a few elongated 

 furrows and ridges nearly parallel with the dorsal or outer margin of the shell; these 



