444 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



tioned, such as the more slender and less digitate branches of the lobes in 

 his species, and the differences in the relative sizes of its third and fourth 

 lateral lobes; but these are by no means greater than we often see in differ- 

 ent individuals of one and the same species, among shells of this kind. 



Externally, however, there are more important differences between 

 these shells; that under consideration having much finer costae, with a 

 decidedly wider umbilicus, showing portions of all tiie inner volutions. The 

 inner or umbilical edge of its outer volution is more rectangular, and merely 

 finely crenate, while there seem to be no traces of small nodes on the sides 

 of any of its volutions. As already intimated, I am not entirely sure that 

 these shells may be more than mere varieties of one species ; but the almost 

 exact identity of their septa does not demonstrate this, because there are 

 clearly distinct species among the Ammonitidee that show scarcely any con- 

 stant differences in the lobes and sinuses of the septa. 



This is another of the Scaphite types, with the deflected part of the 

 outer volution so short, and departing so very slightly from the regular curve 

 of the inner volutions, that it can scarcely be distinguished from Ammonitoid 

 tonus. Hence, Dr. Morton and Dr. Owen both referred it to the genus 

 Ammonites ; and, so far as regards its general aspect, it may be regarded as 

 forming a transition from Scaphites to Ammonites. When we find, however, 

 that it is so closely allied to 8. abyssinus, Morton, both externally and in 

 even the details of its septa, as to leave doubts whether they may not be 

 even varieties of the same species, while in & abyssinus the Scaphite form 

 is sometimes unmistakable (see fig. 4 of pi. 35), and both have septa of the 

 Scaphite type, there seems little or no reason for separating either from that 

 genus. 



As in the case of Scaphites Nicolletii, this form also wants the lateral 

 tubercles of the Discoscaphites section. lis near relations, however, to 8. 

 abyssinus, in which that character does sometimes occur, seems to forbid its 

 separation from the subsection (b) including that species. It seems to bear 

 very nearly the same relations to <S. abyssinus that S. Nicolletii bears to the 

 form I have ranged as var. intermedins of 8. Conradi. 



Locality and position. — Dr. Morton's type-specimen of this species came 

 from the same locality as the last. Those here described and figured came 

 from Moreau River, Dakota, where they occur in the Fox Hills group of the 

 Upper Missouri Cretaceous series. 



