170 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA, [bull. 106. 



u Septa with 12 lateral lobes and as many sinuses on each side, in 

 large examples crowded and very complex." [The detailed descrip- 

 tion of the septa is omitted, as it is more clearly expressed by the 

 figure. | 



The species has hitherto been regarded as characteristic of the Mon- 

 tana formation and its equivalents, and I have been in doubt whether 

 the specimens from the earlier beds should be referred to the same spe- 

 cies, but the scanty material at hand does not show enough difference 

 to permit of separation. Whether they are placed in a different spe- 

 cies or not, I have no doubt that they were the direct ancestors of the 

 form that is so abundant and well developed in the overlying formation. 



The collection under examination consists of single fragments from 

 Huerfano park and Eattlesnake butte, Colorado, and Coalville, Utah, 

 and several more or less fragmentary specimens from Upper Kanab 

 valley, Utah. None of these indicate a size of more than 8 or 10 inches 

 in diameter. The only differences thafc have been observed between 

 these and typical examples of the species is that the septa are some- 

 what less complicated and less crowded, and the periphery seems to 

 have become rounded in somewhat smaller specimens, but the species is 

 known to vary in these respects. A specimen from Ellis county, Texas, 

 where it is associated with Buchiceras swallovi, probably in the Eagle 

 Ford shales, is still more closely like the typical form. 



In the collection from Upper Kanab some examples are smooth, while 

 others, such as the small specimens figured, show a tendency to become 

 nodose, like the variety intercalare. The septum figured was visible 

 only to the edge of the umbilicus, so that several of the small inner 

 lobes and sinuses are not shown. 



Genus PKIONOCYCLUS Meek. 



This generic name was first suggested by Prof. Meek l in 1871, but 

 he then gave no diagnosis, and he did not again use the name in his 

 writings until 1876, when he published 2 a description of the genus and 

 of the two subgenera, Prionocyclus (typical) and PrionotropiSj with P. 

 wyomingensis as the type of the former, and P. woolgari of the latter. 

 In the same work the genus Mortoniceras was proposed with Ammon- 

 ites vespertinus (—A. texanus) as the type. Meanwhile Keumayr had 

 proposed 3 the name SchloenbacMa for the "very natural group of the 

 Cristati," in which he evidently included forms that are referable to 

 Prionocyclus and Mortoniceras, if not to Prionotropis, and in ZittePs 

 Handbuch der Paleontologie these three names are treated as synonyms 

 of SchloenbacMa. They certainly have some features in common, the 

 septa, especially, being similar in all our American species. 



' Ann. Rept. IT. S. Geol. Sur. Terr, for 1870, p. 298. 



2 IT. S. Geol. Sur. Terr., vol. ix, pp. 452, 453. 



8 Deutsch. geol. Gesellscb., Zeitsclir., Ed. xxvn, 1875, p. 887. 



