October, 1841.] 107 



sequently Mr. Nuttall brought some additional species, but for the 

 most part in fragments. Very lately, however, Mr. J. N. Nicollet 

 having personally visited that remote region, obtained a series of 

 fossils in far greater perfection and variety than any previous tra- 

 veller. It is proposed on the present occasion to indicate the 

 species, and accompany them with a few brief remarks. 



Genus Ammonites. 



1. A. mandanensis. Shell compressed, with scarcely two volutions, 

 the ioner being received into a superficial fossa of the outer whorl- 

 Internal and external margins armed with pointed tubercles, 

 between which are delicate, gently curved costas, mostly bifur- 

 cated about one-third of the distance from the outer tubercles, 

 beyond which they extend across the periphery of the shell ; the 

 latter gently plano-convex. Umbilicus imperforate ? 



The diameter of the largest specimen has been about three 

 inches ; of the smallest I have seen, an inch and a half. In the 

 smaller specimeus the internal marginal tubercles are very indis- 

 tinct ; but in other respects this species appears to be but little 

 modified by age. 



J. A. abyssinus. Whorls convex, making two nearly complete 

 volutions, with strong, gently curved, bifurcated ribs, slightly 

 tuberculated at the margin of the dorsal periphery, which they 

 cross to meet the costse of the opposite side. Umbilicus perfo- 

 rate. Diameter from three-fourths of an inch to one inch. 



This species is strikingly different from the A. mandanensis, in 

 the greater size of its costae, its perforate umbilicus, and its convex 

 dorsal periphery. 



3. A. borealis. Shell convex, rapidly enlarging towards the 

 mouth, with at least two volutions, one received deeply into the 

 other ; costae delicate, gently curved, and bifurcate towards the 

 convex dorsal periphery, which they cross in arched lines 

 between numerous minute tubercles. 



Some years since I saw several specimens of Ammonites which 

 were obtained by Judge Bry, in the township of Wachita, in Lou- 

 16 



