FOSSIL SHELLS, &c. 



AROA LIMULA. Tab. I, fig I. 



Oblong, sinuous, rather thin ; ribs numerous, crossed by striae, 

 which are equally distinct in the interstices ; ribs double on 

 the posterior side where they alternate with fine lines; umbo 

 angulated behind ; hinge area narrow, oblique, and transversely 

 striated ; basal margin contracted near the middle ; inner mar- 

 gin crenate. 



Localities. Newbern, N. C. common ; Mr. Nut tall. James 

 River, Va. very rare. Upper Tertiary . 



This shell has a greneral resemblance to Area ponderosa, of 

 Say, but cannot be confounded with that species. 



ARCA TRANS VERSA. Tab. 1, fig. 2. 



Subrhomboidal, rather narrow and thin, with about 32 ribs ; 

 area very narrow, with two or three undulated grooves ; series 

 of teeth nearly rectilinear; within sulcated ; margin crenate. 

 Syn. ARCA TRANSVERSA, Say, Journ. A. N. S. v. 2, p. 269. 



Localities. Newbern, N. C. Mr. Nuttall. Suffolk, Va. in 

 the Upper Marine. Near the mouth of the Potomac, in the 

 Crag. 



This species still inhabits our coast. The fossil specimens 

 are larger and more variable in shape than the recent, except 

 in that deposit which I term Crag, whenever its strata are al- 

 luded to. The species is here abundant and exactly resembles 

 those found upon the beach. Mr. Nuttall favoured me with 

 the specimens represented in the plate. 



ARCA STILLICIDIUM. Tab. I, fig. 3. 



Subcordate, inequivalve and rather thick, with about 30 flat- 

 tened ribs, crenulated on the larger valve ; ribs of the opposite 



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