OF THE UNITED STATES. 37 
This species (the largest hitherto observed in America) 
was described by Dr. Dekay from a fragment, in the se- 
cond volume of the “ Annals of the New York Lyceum of 
Natural History.” By some unaccountabie mistake the 
description there given refers to the wrong figure in the 
accompanying plate: thus, fig. 2 of plate v. is the A. 
placenta, whereas the text refers to fig. 5, which is in 
reality the A. Aippocrepis: this unfortunate error led me 
into the mistake-of calling the former species by the lat- 
ter name; as will be seen in my papers in the Journal of 
the Academy, vol. vi. pp. 88 and 113, and pl. v. fig. 4. 
My description and drawing in that work, therefore, re- 
fer to A. placenta. 
Great numbers of this fossil were found in excavating 
the deep cut of the Chesapeake and Delaware canal. A 
fine specimen, about fifteen inches in diameter, has been 
deposited in the collections of the Academy, by Mr. 
Hugh Lee: and the same gentleman has presented an- 
other to the American Philosophical Society, eighteen 
inches in diameter. It has also been found in many parts 
of New Jersey ; sometimes with portions of the shell re-. 
maining. A similar specimen is contained in the collec- 
tions of the Academy, from the southern bend of the 
Tennessee river. 
2. A. Delawarensis, (S. G. M.) pl. ii. fig. 5. 
American Journal of Science, vol. xviii. pl. ii. fig. 4. 
Specific character. Volutions uncertain; each whorl fur- 
nished with elevated transverse ridges, which bifurcate about 
half way across, and terminate in prominent tubercles on the 
