FOSSIL SHELLS, &c. 
ARC A LIMULA. Tab. \, fig 1. 
Oblong, sinuous, rather thin; ribs numerous, crossed b} r 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. Nuttall, James 
River, Va. very rare. Upper Tertiary. 
This shell has a general resemblance to Area ponderosa , of 
Say, but cannot be confounded with that species. 
ARCA TRANSVERSA. 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. Arc a 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, erenutated on the larger valve ; ribs of the opposite 
