44* UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



lateral branch less on each side ; third lateral sinus shorter than the second, 

 and bearing about the same relations to it in its branchings that the second 

 lateral lobe does to the first ; third lateral lobe about two-thirds as long, 

 and nearly as wide as the second, with three subequal, spreading, and digi- 

 tate terminal branches; fourth lateral sinus less than half as large as the 

 third, oblique, and usually tripartite at the end, the branches being nearly 

 simple. Beyond this there are one or two very small, oblique, nearly simple 

 lobes near the umbilical margin. 



As I only know small, imperfect specimens of this species, which may, 

 in the adult larger examples, develop very different characters, I have not 

 the means of making very satisfactory comparisons with published forms 

 from other localities. 



It is perhaps hardly necessary to add here, that this shell is not a true 

 Ammonites, as that genus is properly restricted ; and that it is here only 

 referred to that group as understood in its old comprehensive signification. 

 This is done merely because its characters cannot be made out, from the few 

 imperfect known specimens, with sufficient clearness to determine the limits 

 and development, of the group to which it belongs. It is evidently not more 

 nearly related to Ammonites proper than others I have placed under other 

 names ; indeed, not as nearly so as one I have referred to Mortoniceras. 



Locality and position. — Both the original type-specimen, and that from 

 which our figures were drawn, were found at the Great Bend of the Missouri, 

 below Fort Pierre, at the base of the Fort Pierre group of the Upper Mis- 

 souri Cretaceous series. Mr. G-abb also cites it from New Jersey ; and I 

 have seen a single specimen, seven or eight inches in diameter, from Miser 

 station, on the Union Pacific Railroad, Wyoming, that appears to be nearly 

 allied to this species. It seems to be a rare shell in the Upper Missouri 

 country. 



Genus MORTONICERAS, Meek. 



Si/non. — Ammonites (sp.), of anthers; not of Brnguiere, as restricted. 

 Etym. — Dedicated to Dr. Samuel George Morton, deceased. 

 Type. — Ammonites vespertinus, Morton (= A. Texanus, Koemer).* 



Shell discoid ; periphery with a simple, low, central keel, and a more or 

 less defined sulcus on each side of it; the sulci being generally each mar- 



*Mr. Gabb, who has compared specimens of A. Texanus, Koemer, from the same locality as Mor- 

 ton's original type of his A. vespertinus, says that they are in all respects identical, Morton's figure being 

 very defect ive. 



