Logan.] The Invertebrates, Benton Group. 467 



" Young examples, half an inch to one inch in diameter, with 

 costse linear, closely arranged, of nearly uniform size, and mani- 

 festing scarcely any tendency to develop nodes, but already 

 showing the forward curve of their outer ends well defined, 

 while the peripheral keel is low, narrow, and simple, and the 

 furrow on each side shallow. At a somewhat larger size, costse 

 usually more or less unequal in size, the larger ones now begin- 

 ning to develop the two nodes at their outer curved ends, and 

 to become a little more prominent and compressed at their inner 

 extremities, while the rather more prominent keel begins to 

 develop its crenate outline, and the nodes nearest it to assume 

 their compressed form and parallel arrangement. On attaining 

 to two and one-half to three inches in diameter, costae, nodes, 

 and keel becoming more prominent, the latter being strongly 

 compressed and deeply and largely scalloped, with divisions 

 rounded in outline ; while, at this stage of growth, the periphery 

 as seen in profile would seem to be very deeply sulcated on each 

 side of the keel, but this is due to the prominence of the row 

 of nodes on either side of the same. Costse, when the shell 

 has attained a diameter of four inches, much depressed in the 

 middle, with the nodes at their inner ends thicker and more ob- 

 tuse, and those nearest the keel more depressed or nearly ob- 

 solete, 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 represented by distinctly separated, low, 

 elongated nodes ; and when the shell has attained a diameter of 

 seven inches, the costee are more distant, greatly elevated, com- 

 pressed, 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 either side, the two 

 terminal of which are largest, more or less parallel, and merely 

 serrated ; first lateral sinus broader than the siphonal lobe, more 

 or less deeply divided into two subequal branches with short, 



