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, /. Sowerby. Tab. XXX, fig. 1, a — d. 



Pholadomya candidoides. 5. Wood. Catalogue, 1840. 



— hesterna. J. Sow. Min. Conch., t. 629, 1844. 



Spec. Char. Testa transversa, ovato-oblongd, aut irregulariter pyriformi, valde inaqui- 

 laterali ; antice ventricosd, rotundatd vel subtruncatd ; postice productd ; mediand parte 

 cost at d, cost is 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, 2^ 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, Ph. 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 Anatina. 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, b, figs. 1 — 8, resembles our shell, judging from the figure, but appears to 

 have had too many ribs. Ph. Esmarkei (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 cither, and have therefore retained Mr. Sowerby's name. 



