THE LAMELLIBRANCFIIA 71 



by species throughout the Jurassic, became less common in 

 the Cretaceous, is still represented by several species in 

 the British Eocene beds and one in the Pliocene, but only 

 survives to-day in deep waters in the Atlantic Ocean and 

 West Indies, although many of the fossil species (even up 

 to the Pliocene) must have lived in shallow waters. We 

 may take P. fidicula of the Inferior Oolite as an example 

 (Fig. 22). 



The shell is extremely thin, so much so that when 

 found as an internal cast, it requires careful examination 

 to determine whether any of the shell is still adherent or 



FIG. 22. PHOLADOMYA SP. RESEMBLING BUT SHORTER THAN 



P. FIDICULA (J. DE C. SoWERBY), AALENIAN. ( X .) 



Left side view. (After Goldfuss.) 



not. Internally it is nacreous. The shell is equivalve, 

 and strongly inequilateral, the umbones being far forward 

 and prosogyral. The valves gape at the posterior end, 

 the long siphons not being fully retractile ; consequently 

 the interior is always filled with rock-matrix and the 

 posterior end has a rough, broken appearance where the 

 matrix within joined that without. The surface is 

 ornamented with numerous oblique, slightly curved, 

 radiating ribs, dying away in the postero-dorsal area. 

 (In some species the ornament is more elaborate, with 

 fewer an.d coarser ribs and tubercles, and differing in 



