AMMONITES BILLINGSI. 



315 



from any of the described species from the Nebraska 

 rocks, though I think I have seen some fragments of it in 

 Lieut, Warren's collections from No. 4 of the Nebraska 

 cretaceous subdivisions. 



The specific name is given in honour of Mr. George 

 Barnston, chief factor of the Hudson's Bay Company, 

 who discovered it in the valley of Mackenzie's Eiver. It 

 is probably a Cretaceous species, but may be of Jurassic 

 age.] 



A mmonites Billingsi. 



Fig. 1. Fig. 2. Fig. 3. 



Fig. 1. — Side view. Fig. 2. — One of the septa enlarged. 

 Fig. 3.— Front view of same specimen. 



[Shell moderately compressed, or subdiscoidal ; dorsum 

 rounded ; umbilicus very small ; volutions having their 

 greater breadth at right angles to the shorter diameter of 

 the shell, increasing rather rapidly in size, or more than 

 doubling their diameter each turn ; inner ones entirely 

 embraced, and hidden in the ventral groove of the last 

 turn : surface apparently smooth, but showing very faint 

 traces of radiating costal, which arch a little in crossing 

 the dorsum. 



Dorsal lobe longer than wide, provided with three 

 branches on each side, the two terminal of which are 

 much longer than the others, and each subdivided, — the 

 subdivisions being short, and each having two or three 



