INVERTEBRATE PALEONTOLOGY. L53 



on (lit 1 last turn of large adults, the outer compressed node sometimes 

 becomes obsolete, and the other near it, much enlarged, or produced as a 

 short, thick spine. 



Septa with about three lateral lobes on each side, the first of which is 

 longer than the siphonal lobe and tripartite at the end, while the others are 

 much smaller and trifid, or the middle one sometimes bifid ; first lateral sinus 

 broad and bilobed, the outer lobe lapping partly upon the peripheral side. 



The above diagnosis is intended to include two types, which would, 

 according to the most restricted of the late classifications of such shells, prob- 

 ably represent two distinct genera, but would doubtless .be both included in 

 one group by European authors, who have of late years undertaken the sub- 

 division of the old genus Ammonites. For the present, at least, I prefer to 

 treat these two types as sections of one genus They may be separately 

 defined as follows : 



1. prionocyclus, Meek (typical). 



Shell, when very young, with costse obscure, but soon after 

 becoming well developed, the larger tuberculated ones being separ- 

 ated by several smaller, usually without tubercles, even on the last 

 turn of large adults, where none of them become greatly increased 

 in size, or develop large tubercles; keel continued prominent, and 

 not broken up into isolated nodes on any of the volutions. — (Type as 

 already stated.) 



2. prionotropis, Meek. 



Shell, when very young, with costse sharply defined, and as the 



whorls increased in size, becoming more distant, without having the 



intervening spaces occupied by smaller ones; on the last turn, costae and 



their nodes becoming very prominent, and the keel depressed and 



broken into a series of isolated, elongated nodes. — (Ammonites Wool- 



gari, Mantell.)* 



The type-species of this genus so nearly resembles some species of 



Professor Hyatt's genus Plcuroceras (not Pleurocerd, Hat'.), that I have been 



much inclined to think it might belong to the same, although that genus was 



founded entirely on Liassic species. On comparison with Ammonites spina' us, 



' I cite Mantell'a species us the type of this group, because I believe the following-described form 

 to be ideutic.il with the same. As I know nothing, however, of the septa of Mantell'a type, it is still 

 barely possible that it may In- distinct from our shell, in which case the latter, from which the foregoing 

 description was drawn up, would be the type of the group. 



