452 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



It is on the supposition that this shell is a young example, in which the 

 usual characters of the group Mortoniceras have not been fully developed, 

 that I here refer it to that group. It might even be a young of the last, 

 or of M. vespertinum. 



It is evidently not a young specimen of Prionotropis Woolgari, as its 

 costse are broader and decidedly straighter than those of that species of the 

 same size, and show no traces of the double node usually seen at their outer 

 ends, even in smaller examples of the same ; while its keel is also without 

 the obscure crenulations of that shell, and its volutions less compressed. It 

 is true that the crenulations of the keel in specimens of P. Woolgari of this 

 size are not well developed ; but traces of them can usually be seen near the 

 larger extremity of the outer turn at that stage of growth. 



Locality and position. — Mouth of Vermilion River, Nebraska, on the 

 Missouri; where it was found in the Fort Benton group of the Upper Missouri 

 Cretaceous series. 



Genus PRIONOCYCLUS, Meek. 



Synon. — Ammonites (sp.), of authors; not of Bruguiere, as restricted to typical forms. 



Prionocnjelus, Meek (1872), in Haydeu's Ann. Report U. S. Geological Survey of tbe Territories, 

 298 (foot-note). 



Etym. — TTfituv, a saw ; kvkXoc, a circle ; in allusion to the circular form of the shell and its serrated keel. 

 Type. — Ammonites scrrato-carinatus, Meek (not Stoliczka).* 



Shell discoid, with periphery more or less depressed, and provided with 

 a central keel defined by a shallow depression or concavity on each side ; 

 keel in the young simple, but at a later stage of growth strongly crenate, 

 very prominent and compressed, and in large adult shells again depressed, or 

 sometimes merely represented near the aperture by a row of separate, elon- 

 gated nodes ; umbilicus open; volutions more or less compressed, and but 

 slightly embracing; surface costated and tuberculated ; costae in the young 

 simple, smooth, and strongly curved forward as they approach the keel, near 

 which, at a later state of development, they each, or in part, produce two 

 nodes, the outer of which is compressed, with its longer diameter parallel to 

 the keel, while the umbilical end projects so as to form a compressed node; 



* At the time I indicated this species in one of Dr. Haydeu's reports (1872), I was not aware that 

 Dr. Stoliczka had,previonsly used that name for a similar Indian species. I know nothing in regard to 

 the septa of his species ; hut, from its external characters, il seems to belong to the same genus as that 

 for which I had used the name A. scrrato-carinatus. If so, it will be necessary to designate the American 

 shell by another specilic name ; in which case I would propose to call it Prionocyclus IVyomingcnsis, since 

 it is the most abundant and largest shell of the kind yet known from that Territory. 



