MIOCENE MOLLUSC A AND CEUSTACEA. 29 



White, in his Non-Marine Molluscan Fauna of the West, l and in his work 

 on the Fossil Ostreidae of the United States. 



Formation and locality: Fossil in the Miocene near Shiloh and at 

 Elwell's marl pits at Jericho, N. J. 



OSTREA PERCRASSA. 



PL in, tigs. 1-4. 



Ostrea percrassa Conrad: Medial Tert. Foss., p. 58, PL xxv, fig. 1. Meek, Smith 

 Check List, p. 3. White, Fam. Ostreidae; p. 313, PL lxviii, fig. 3. 



Shell rather large, very thick and heavy in appearance; subcircular in 

 outline, or obscurely broad subovate, being widest in front of the middle 

 of the length; the lower valve often highly convex on the outside and deep 

 within. Hinge-area large and strongly lamellose, striate transversely; 

 ligamental groove broad and deep. t Upper valve less deep than the lower, 

 also less convex on the outer surface. Muscular scars very large, semi- 

 lunate or semi-elliptical; extending far beneath the shell at the back edge, 

 forming a deep cavity beneath it. Margin of the shell outside of the pallial 

 area smooth. Substance of the shell very strongly lamellose, the lamellae 

 being separated by a minutely vesiculose interlamellar substance, as in 

 GrypJioea vesicularis Lam. 



The New Jersey forms of this shell which have come under my notice 

 are not nearly so large and ponderous as those which I have seen from 

 South Carolina. Mr. Conrad's figure given in his Medial Tertiary fossils is a 

 very fair representation of the general run of the examples from New Jersey. 

 While many of those from the more southern localities have grown longer 

 with advanced age, but not wider in the same proportion, still one or two 

 examples from the Santee River in South Carolina, now in the collection of 

 the American Museum of Natural History in New York, are very broad and 

 not so thickened; while the outer surface of the upper valve is almost flat 

 and comparatively smooth. A peculiar feature of this species, seen in the 

 New Jersey specimens but not in those from South Carolina, is the finely 

 vesicular, interlamellar structure, which, when the upper layer of pearly 

 shell is removed from the one beneath, presents a very fine froth-like 



1 Fourth Annual Rept. U. S. Geol. Surv. 



