82 EOCENE MOLLUSCA. 
4. Arca prepressa, J. Sowerby. Tab. XIV, fig 4, a—e. 
Arca pEprmSSA. J. Sow. Min. Conch., t. 474, fig. 2, 1824. 
— — Morris. Catal. Brit. Foss., p. 185, 1854. 
Spec. Char. A. testa tenui, elongato-oblongd, valde inequilaterali, depressd, radiatim 
striata et concentricé 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, ths of an inch; eight, } an inch. 
Localities. Woolwich (Sowerby), 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 distine- 
tion appears to be its depressed form, or shallow valve; the stria upon the exterior Mr. 
Sowerby describes as “ very distant upon the anterior side (siphonal region ?), and appear 
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. 
” 
like small knotted threads ; 
5. Arca Dunwicurensis, Edwards, MS. Pl. XV, fig. 6, a, 4. 
Spec. Char. A. testd ovato-oblongad, subobliqud, gibbosuld, valdé inequilaterali, 
utrdque eatremitate late obtusd, in medio depresso-sinuosd, radiatim costellatd ; costulis 
angustis prominulis, in siphoni-regione 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, \2 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 d4rca. 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 
