456 UNITED STATES GEOLOGICAL SURVEY OE THE TERRITORIES. 



inches, much depressed in the middle, with the nodes at their inner ends 

 thicker and more obtuse, and those nearest the keel more depressed or nearly 

 obsolete, while those of the third series, near by, become much enlarged 

 and produced obliquely outward as short, thick, spine-like projections. 

 Soon the outer compressed nodes disappear, and the keel is only repre- 

 sented by distantly-separated, low, elongated nodes ; and, when the shell 

 has attained a diameter of seven inches, the costas are more distant, greatly 

 elevated, compressed, and almost wing-like, but still retain a large, promi- 

 nent, subtrigonal node, or projection, at their outer ends, and again become, 

 as it were, pinched up at their inner extremities, which do not quite reach the 

 umbilical margin. 



Septa moderately close together ; siphonal lobe longer than wide, with 

 three or four short branches on each side, the two terminal of which are 

 largest, more or less nearly parallel, and merely serrated ; first lateral sinus 

 broader than the siphonal lobe, more or less deeply divided into two sub- 

 equal branches, with short, irregular branchlets and digitations ; first lateral 

 lobe somewhat longer than the siphonal and tripartite, with short, irregu- 

 lar branchlets and digitations — occasionally, in small specimens, with the 

 middle terminal branch proportionally broad and so deeply sinuous at the 

 end as to impart more nearly the appearance of a bipartite arrangement of 

 the whole; second lateral sinus nearly resembling one of the divisions of the 

 first, and, in the adult, with merely a number of marginal digitations; second 

 lateral lobe little more than one-third as long, and from one-third to one-half 

 as wide as the first, generally tripartite at the end, but sometimes, in large 

 specimens, bipartite on one side of the shell (see fig. 1, h, of our plate 7), 

 the divisions being very short and simple, or serrated ; third lateral sinus 

 very small and merely bilobate, or in large specimens digitate along the 

 margins ; third lateral lobe hardly half as long as the second, and in small spe- 

 cimens (it has not been seen in the large ones) merely tridentate at the end. 



Largest specimen seen (with a part of the non-septate portion wanting), 

 7 inches in its greatest diameter ; convexity, measuring between the costse 

 at the larger broken end of the last turn, 1.60 inches; convexity of the same, 

 measuring so as to include the greatly expanded costse, 3.25 inches. 



In regard to the different features developed by this shell at different 

 stages of its growth, already described, it is probably hardly necessary to 

 remark that the changes mentioned arc nut at any point abrupt, but that the 



