CRETACEOUS FOSSILS. 137 



A. FRATERNUS, D. 8. 

 PI. 23, Fig. 15, a, b. 



SHELL small, whorls rounded, deeply enveloping; sides rounded 

 and converging towards the dorsum, which is regularly convex; 

 umbilicus deep, moderate in size; umbilical margin rounded. 

 Surface marked by slightly sinuous ribs, nearly straight, arising 

 on the umbilical margin, and continuing over the dorsum; every 

 fourth or fifth of these ribs is a little larger than the others, and 

 bears a prominent flattened tubercle on the border of the umbil 

 icus. An occasional constriction exists between the ribs. 



Septum composed of a broad dorsal, a large superior lateral, a 

 very small inferior lateral, and two supplementary lobes. Dorsal 

 divided for about a third of its length, carrying two large ser 

 rate teeth over the siphuncle, each branch with a large, and two 

 smaller spurs on the outer side ; body of the lobe with three large 

 spurs above the division. Dorsal saddle bifurcate, each branch 

 subdivided. Superior lateral lobe trifurcate, the branches nearly 

 equal and trifid at the extremities ; a large and a small spur above 

 the fork. Lateral saddle nearly as wide as the dorsal. Inferior 

 lateral lobe not more than half as long as the superior, trifid on 

 the end, with two small spurs on the upper side and one on the 

 lower. Supplementary lobes very small and trifid. 



Diameter, 1.4 inch; diameter of umbilicus, .45 inch ; height of aperture, .55 

 inch ; width, .6 inch. 



A single specimen, nearly a miniature of A. Traskii, in general appearance, but 

 with almost identically the septum of A. Hoffman-nil, differing only from that of 

 the latter species in some of the details. It cannot be the young of either of 

 these species however, because though its surface resembles Traskii the septum is 

 radically different; while all of its external details, ,he form and ornamentation, 

 are entirely unlike any of the very large number of Hoffmannii that I have ex 

 amined. No specimen of that species that I have ever seen shows the slightest 

 trace of tuberculation, and in none are the ribs so marked. The single constric 

 tion on the surface of the specimen before me, is not due to a thickening of the 

 shell-substance, but is as strongly marked on the outside of the shell itself as on 

 the cast. Another point, though of less importance than the above, is that this 



PAL. VOL. II. 19 



