266 MOLLUSCA FROM THE CRAG. 
Pholadomya is a genus of great geological antiquity, being known as early as the 
Coal Measures, and continued to the present time through the Secondary Periods, 
where it was largely developed, presenting only a few species in the Tertiaries. 
PHOLADOMYA HESTERNA, J. Sowerby. Tab. XXX, fig. 1, a—d. 
PHoLADOMYA CaNDIDOIDES. S. Wood. Catalogue, 1840. 
_ HESTERNA, J. Sow. Min. Conch., t. 629, 1844. 
Spec. Char. Testd transversd, ovato-oblongd, aut trregulariter pyriformi, valde inequi- 
laterali ; antice ventricosd, rotundatd vel subtruncatd ; postice productd ; mediand parte 
costatd, costis 10 —12. 
Shell transverse, ovately oblong or of an irregular pear-shape, very inequilateral 
anterior side ventricose, rounded, or slightly truncated; posterior much produced ; 
centre covered with about 10 or 12 ribs. 
Length, 4 inches. Height, 25 inches. 
Locality. Cor. Crag, Ramsholt and Sudbourn. 
This species, as might be expected, is found only in beds which have undergone no 
disturbance ; and at Ramsholt, where these shells are quietly deposited in the sand, I 
have seen many specimens, but they are exceedingly difficult to obtain, and then with 
only a small portion of its thin shell remaining. Like those from some parts of the 
London Clay, the specimens are in general much distorted. Our figure is from one 
that has preserved its natural form ; and although there is a considerable resemblance 
to the only living species that has as yet been discovered, it seems to be specifically 
distinct. The recent shell, P2. candida, is more tumid centrally, and less rounded on 
the anterior side, while on the other it is not so broad, and there is a greater curve in 
the ventral margin. Our shell appears to have been closed on the anterior side, but 
had a gape or opening for its probably lengthened siphonal tubes. The hinge is 
furnished with a small obtuse tooth in each valve, and a marginal lamina or fulcrum 
for the ligament, this is bipartite, separating slightly the cartilage from the ligament ; 
the umbones, like those of the recent shell, approximate so closely as to have been 
fractured by the opening of the valves. The ribs of our shell extend over rather more 
than half the surface, from the obtuse ridge on the anterior side to within about the 
same distance from the other extremity, and they are covered with small obtuse tuber- 
cles ; while the small portions of the shell that are remaining present a finely granulated 
surface, or shagreen, like those of Azatinma. This is especially visible near the siphonal 
extremity, but was probably more or less granular all over. Ph. arcuata, Agass.,‘ Etud. 
crit.,’ p. 63, t. 2, 4, figs. 1—8, resembles our shell, judging from the figure, but appears to 
have had too many ribs. PA. Hsmarker (Pusch), Goldf., ‘Petr. Germ.,’ vol. ii, p. 272, 
t. 157, fig. 10, a—d, may probably be the same. I have been unable to examine 
specimens of either, and have therefore retained Mr. Sowerby’s name. 
