COILOPOCERATID ®. 91 
COILOPOCERAS ® n. gen. Hyatt. 
This genus includes species formerly associated with Sphenodiscus, 
which, however, differ so that they are easily separated. The development 
and form are closely similar; that is to say, the shell develops directly 
from the rounded nepionic volutions into a helmet-shaped whorl in section 
having subacute venter which becomes more acute with age. In the three 
forms described below in the neanic and adult stages, the volutions are 
somewhat less compressed than in species of Sphenodiscus, but this is prob- 
ably only a specific character. 
The sutures differ in having broad first lateral saddles with a peculiar 
large internal arm or marginal saddle, and the first and second laterals, when 
the second are present, are narrow and very distinct from the first laterals 
on the outer side and the first large auxiliary saddles on the inner side. 
There are but two secondary lateral saddles, the first and second, derived 
from the division of the primitive first laterals in C. colleti, the type of the 
genus, but this form is in an early ephebic substage and the aspect of C. 
novimexicanum at a somewhat older stage shows that there is an approxima- 
tion to the trisellate condition. This trisellate or placenticeran aspect of 
the principal or first outer saddles, derived from the division of the primi- 
tive first lateral saddles, is still more marked in C. springeri. The hollow 
keel is remarkably distinct. When the shell is removed, the siphuncle is 
covered only with a very thin layer of the fossilizing sediment, and this 
organ is apt to be exposed on the casts and is often absent in more or less 
worn specimens, leaving the venter deeply channeled. This last accident 
is of rare occurrence among forms with solid keels. The siphuncle is also 
larger and thicker walled than in Sphenodiscus. 
CoImLoPocERAS COLLETI n. sp. Hyatt. 
Pl. X, figs. 5-21; Pl. XI, fig. 1. 
This species is easily distinguished by comparison with its nearest 
affine, Coilopoceras novimexicanum. It has stouter volutions, a more open 
umbilicus, the venter is blunter, and in the young, at any rate, it has larger, 
stouter fold-like costae, one long one reaching to the umbilical shoulder, 
«a Kowlwros, hollow. 
