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 unaccountable 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. hippocrepis : 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 pi. v. ^§. 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.) pi. ii. fig. 5. 



American Journal of Science, vol. xviii. pi. ii. fig. 4. 

 Specific character. Volutions uncertain; each w T horl fur- 

 nished with elevated transverse ridges, which bifurcate about 

 halfway across, and terminate in prominent tubercles on the 



