460 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



simple lateral branchlet on the other side; third lateral sinus about half as 

 large as the second, and unequally bipartite at the end, nearly in the same 

 way as the first, but with all its divisions much shorter and less deeply sub- 

 divided and sinuous ; third lateral lobe very much smaller than the second 

 on one side of the type-specimen, and bifid at the end, and on the other 

 more slender and distinctly tridigitate ; fourth lateral sinus very small, and 

 merely bi- or trilobate ; fourth lateral lobe as seen on one side of the type- 

 specimen scarcely half as large as the third, with merely slightly digitate 

 margins, and a more or less truncated or indented extremity ; fifth lateral 

 sinus (not shown in the septum figured on our plate) broader than long, and 

 equaling the length of the fourth lobe, with its broadly-truncated end den- 

 tate; fifth lateral lobe (not shown in the figure on the plate) about half as 

 long and wide as the fourth, oblique, and showing a tendency to become 

 tridentate at the end. 



A comparatively small specimen, with perhaps at least half of its outer 

 volution broken away, measures 4.70 inches in its greatest diameter, and 

 about 1.50 inches in convexity. Another crushed specimen shows that it 

 attains a diameter of twelve inches or more. 



It is not without considerable doubt that I have ventured to refer this 

 shell to the genus Phylioceras, although there seems to be good reason, as 

 already suggested, for believing that group to have been represented during 

 the deposition of the Cretaceous as well as the Jurassic rocks. In general 

 form (at least in medium-sized specimens), as well as in the roundness of its 

 periphery and the general characters of its septa, it agrees pretty well with 

 that genus. Its septa, however, differ in some details, such, for instance, as 

 the proportionally smaller size of its first lateral lobe as compared with the 

 siphonal lobe, and the more nearly bipartite termination of the former; also, 

 in the less obtusely-rounded terminations of the subdivisions of the lateral 

 sinuses. As already remarked, however, the difference between the size of 

 the siphonal and first lateral lobes is not generally so great as in the septum 

 represented on our plate; while, on the opposite side of the same specimen 

 from which that drawing was made, the first lateral lobe shows more of a 

 tendency to tripartite division, and the little third lateral is distinctly tripar- 

 tite. On scraping away the shell from the internal cast, close in at the inner 

 margin of the volutions, after the diagram of the septum on our plate 24 was 

 lithographed, there was found a very small fifth lateral sinus, wider than 



