

442 UNITED STATES GEOLOGICAL SURVEY OE THE TERRITORIES. 



breadth, with two branches on each side, the two terminal of which are of 

 moderate length, spreading, bifid, and provided with obtusely crenate or den- 

 tate margins, while the two lateral branches are much smaller, directed out at 

 right angles from the margins, and each obtusely tridentate at the end ; second 

 lateral sinus scarcely half as large as the first, and bearing two unequal, 

 shortly bipartite or tripartite, deeply sinuous, and obtusely dentate terminal 

 branches, and one or two unequal, short, nearly simple, lateral branchlets ; 

 second lateral lobe much like the first, but scarcely more than half as long 

 and wid° ; third lateral sinus about one-third as large as the second, with 

 two short, spreading, obtusely dentate, terminal branches; third lateral lobe 

 very small, and merely obscurely bilobate at the end; fourth lateral sinus as 

 long as the third lateral lobe, but wider and faintly bilobate at the end; 

 fourth lateral lobe slightly wider and longer than the third, but more dis- 

 tinctly trilobate at the end. 



Length, 2.09 inches; height, 1.65 inches; convexity, 0.63 inch. 



The angle around the inner side of the non-septate part of the outer 

 volution in this species is not always well marked, and varies a little in its 

 distance from the umbilical margin. It is apparently always provided with 

 a. row of low prominences, like transversely-elongated nodes, that never 

 exist on the inner volutions. The angle on each side of the narrow, flattened 

 periphery, with its row of nodes, seems to be always well defined on the 

 outer volution, excepting near the aperture, where both angles and nodes 

 usually fade away,, and the costse become finer and crowded. On the inner 

 volutions, also, the peripheral angles become obscure or obsolete, though 

 their place is occupied by the rows of nodes reduced in size. The little 

 nodes seen on the sides of the inner volutions, and sometimes on the inner 

 half of the last turn, are placed on the costae so as to form about three nearly 

 equidistant, revolving rows on each side of the shell. 



This form is very closely allied to the following species, and may pos- 

 sibly be only a variety of the same. The points of difference will be noticed 

 in connection with the description of that shell. There is little or no room 

 for doubts in regard to this being the form to which Dr. Morton applied the 

 name Ammonites abijssinus. It is true his type-specimen showed none of 

 the little nodes on the costre of the sides of the shell ; but these are not 

 always present, as may be seen by our figure 2, a, and those near the margin 

 of the umbilicus only occur on the outer volution, while his type-specimen 



