stanton.] AMALTHEID.E. 173 



lobe, but much wider, and deeply divided into two unequal parts, of 

 which the one on the siphonal side is larger than the other, each of 

 these principal divisions being ornamented by some four or five short, 

 irregular bran chlets, with obtusely digitate margins ; first lateral lobe 

 longer and slightly wider than the siphonal, and provided with some 

 seven or eight short, rather unequal, merely digitate, and palmately 

 spreading terminal and lateral branchlets; second lateral sinus nar- 

 rower, but as long as the first on the outer or siphonal side, and much 

 shorter on the umbilical, having two short, unequal, digitate, terminal 

 branches at the end, and some three or four short, irregular divisions 

 along the oblique margin of the umbilical side 5 second lateral lobe 

 small, or scarcely more than twice as large as the auxiliary lobe of the 

 siphoual sinus, and somewhat irregularly bifid, the divisions being 

 short, and, like the lateral margins, more or less digitate. 



" Greatest diameter of a specimen retaining only a small portion of 

 the non-septate outer whorl, 4.40 inches; greatest convexity of same, 

 0.95 inch; breadth of umbilicus, 1.35 inches; breadth of the last whorl 

 from the siphonal to the umbilical side, 1.80 inches." 



The description of the septum above given was made out from a weath- 

 ered specimen that did not show all the details clearly. The one I have 

 figured is drawn from a smaller specimen, and it is therefore not quite 

 so deeply divided as the one originally figured. There are also slight 

 differences in the forms of the second lateral lobe and the second sad- 

 dle, but these are in part due to errors in restoring the septum of the 

 type. It will be seen by the figure now given that the septum is very 

 similar to those of the other species of Prionocyclus above described. 



In the young stages of this species the costse are simple, linear, closely 

 arranged, and strongly curved forward. The keel is then minutely 

 crenate, with one crenation for each costa. After the third volution 

 some of the costse are more strongly developed and bear nodes or small 

 spines at their outer ends while the intermediate ones become obsolete. 

 The crenations of the keel stiJl continue numerous and small. 



One of the specimens^ figured (Fig. 2) has unusually strong spines 

 around the periphery and the larger costae are farther apart than in 

 the other specimens of the same size. 



This species is more closely related to Prionocyclus wyomingensis than 

 to any other described American species. It can be easily distin- 

 guished by its more compressed form, by the greater regularity of the 

 costaa, and by the much greater difference between its young and adult 

 stages. 



Locality and position. — The types came from "Banks of Canadian; 

 lower part of Middle Cretaceous of New Mexican section." I have col- 

 lected it near Mancos, southwestern Colorado, about 400 feet above the 

 base of the Colorado Cretaceous shales, where it is associated with 

 Ostrca lugubris and Inoceramus dimidius. 



