464 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



of Limbourg; also, perhaps, A. Requienanus, d'Orbigny, from the Upper 

 Greensand of France, and probably others. The last-merationed species 

 has exactly the form and surface-characters of the type of this group, and 

 also shows the undulated character of the rows of lobes and sinuses : but, 

 according to d'Orbigny 's description, it would only have six lateral lobes on 

 each side. It is evident, however, from analogy, that what he has described 

 as three large terminal branches of the first lateral (superior lateral) lobe, 

 really correspond to the first three lateral lobes of the type under consider- 

 ation. Consequently, if we count these as three lobes, instead of merely 

 three large divisions of the first lateral lobe, we would at least have eight 

 lateral lobes on each side of this shell, which would bring it very nearly, if 

 not quite, within this group. 



Some of the species included by Mojsisovics in his genus Pinacoceras, 

 published in 1873 in his work on the Cephalopoda of the Zlambach and 

 Halstiitt Shales, seem to be nearly i - elated to this genus, both as to form and 

 the nature of the septa. His P. trochoidcs, for instance, has almost exactly 

 the form of the type of the Sphcnodiscus section of Placenticeras, and, like 

 the types of both sections of this group, it has a large number of lateral 

 lobes, the third lateral being the largest, but still its fourth and fifth laterals 

 are also large; while its siphonal lobe is proportionally much larger than in 

 either of our types. Again, its whole series of lateral lobes, instead of run- 

 ning in an undulated line across the sides of the volutions, form one continu- 

 ous, broad, forward sweep, all the way across. Others of the species illus- 

 trated by him approach more or less nearly our group, but present similar or 

 other differences in the nature of the septa. It seems to me that his group 

 Pinacoceras is made too comprehensive, or, in other words, includes forms 

 representing several generic groups, strictly speaking. That any of his 

 included species, however, if all their characters and their development from 

 the young to the old are considered, would fall into our genus, is very improb- 

 able, in view of their widely different geological position. Whatever opin- 

 ions, however, may be entertained on this point, our names, having priority of 

 date, would have to stand. 



My present opinion is that both sections of this genus are confined to 

 the Cretaceous system, and mainly to the upper members of the same. 



