212 Field Columbian Museum — Geology, Vol. i. 



the uncovered nodes forming the inner of the two outer rows on the suc- 

 ceeding volution within; umbilicus about equaling the greatest dorso- 

 ventral diameter of the last turn. Young examples, half an inch to 

 one inch in diameter, with costae linear, closely arranged, of nearly 

 uniform size, and manifesting scarcely any tendency to develop nodes 

 but already showing the forward curve of their outer ends well 

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

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

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

 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 to assume their compressed form and 

 parallel arrangement. On attaining to two and one-half to three 

 inches in diameter, costae, nodes and keel become more prominent, 

 the latter being strongly compressed and deeply and largely scal- 

 loped, with divisions rounded in outline; while at this stage of growth 

 the periphery, as seen in profile, would seem to be very deeply sul- 

 cated on each side of the keel, but this is due to the prominence of 

 the row of nodes on either side of the same. Costae, 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 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 represented 

 by separated, low, elongated nodes; and when the shell has attained 

 a diameter of seven inches, the costae are more distant, greatly ele- 

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

 prominent, 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 subequal branches with short, irregular branchlets 

 and digitations; first lateral lobe somewhat longer than the siphonal, 

 and tripartite, with short, irregular branchlets and digitations occa- 

 sionally in small specimens, with the middle terminal branch propor- 

 tionately broad and so deeply sinuous at the end as to impart more 

 nearly the appearance of a bipartite arrangement of the whole; sec- 



