454 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



Bruguiere, for instance, which Professor Hyatt includes in his genus Pleuro- 

 ceras, our type is found to present the same general features ; the chief differ- 

 ence being that its periphery is less flattened, not so deeply furrowed, and its 

 keel more prominent, while it has another series of smaller costse between 

 those corresponding to the costxe of A. spinatus; and those of the latter 

 also differ in not being tuberculated at the inner ends as in our type. There 

 are also some differences in the septa, but not, I should think, if taken alone, 

 of generic importance. 



Nevertheless, as Professor Hyatt, who has given more attention to the 

 study of the Ammonitoid types than any other person in this country, is quite 

 positive in the opinion that the genera of this great group of shells are very 

 restricted in their geological range, and that probably none of the Liassic 

 groups range up into the Cretaceous, I have concluded to view the differ- 

 ences mentioned as generic, and retain my name Prionocyclus, originally 

 provisionally proposed as a subgeueric name, in a generic sense here. 



The other section, for which I propose here the name Prionotropis, 

 represented by such forms as Ammonites Woolgari, Mantell, is more decidedly 

 distinct in its development, though at certain stages of growth the two forms 

 are less strongly separated than in the adult. These two groups, whatever 

 may be the relations of the first to Pleuroceras, probably represent two dis- 

 tinct genera ; but as I have not at hand a sufficiently complete series of 

 species to be able to characterize them fully as such, I have preferred to 

 keep them together for the present. 



I am not altogether sure that any other species can be cited as belonging 

 to the Prionotropis group than the type, but would merely mention A. 

 Bravamanus and A. Carolinus, d'Orbigny, as very closely resembling speci- 

 mens of our typical form of the same size; also A. serrato-carinati/s of Sto- 

 liczka. The latter, however, if included, would require slight modifications 

 of the diagnosis, as it has deeper furrows on the periphery, and a few addi- 

 tional nodes along the middle of the costse. 



Compared with Ammonites proper, as typified by A. bisulcatus, and 

 restricted to the group including that species, this genus is distinguished by 

 having but one peripheral keel, and that strongly crenated, instead of three 

 smooth keels and two well-defined furrows between them. There are also 

 strongly-markcrl differences in the development of these types, Ammonites 

 proper, in the very young condition, having the costse represented by rows of 



