230 PSEUDOCERATITES OF THE CRETACEOUS. 
early stage on account of the much compressed wholly smooth volution 
and: narrow concave venter with slightly elevated .acute ridges on either 
side on the cast. At this age inner saddles assume a primitive rounded 
form with expanded base and one marginal central lobe; the inner lobes 
are trifid. Two specimens in the collection of the Boston Society of 
Natural History, collected by Prof. W. G. Crosby, have the typical 
characters and form of P. whitfieldi and are devoid of tubercles. The 
sutures were not very clearly made out. The matrix has the aspect 
of an arenaceous ? limestone, color light brown. The largest specimen 
from Fort Collins, Colo., is 185 mm. and the smaller one from E] Paso 
County, Colo., 84 mm. in whole volution. One specimen in the collection 
of the Boston Society of Natural History reaches 265 mm. in diameter and 
has a coneave ventral zone on both shell and cast at end of outer volution. 
There are excessively faint tubercles along the umbilical shoulders but no 
traces of them on the edges of the ventral zone, either on shell or cast. 
The shell on this specimen and on several others is sufficiently perfect 
to show that there are the following parts. An outer opaque probably 
porcellaneous part of several layers, next a middle part with more or less 
of luminous red coloration, also of several layers, and an inner part, also of 
several layers, with the usual iridescence of nacreous shell. 
Two specimens from Loup Fork, Nebr., in the Museum of Comparative 
Zoology, show the typical large siphonal saddle with dependent marginal 
saddles of whitfieldi. One of them belongs to the parephebic stage of a 
shell having at the same time concave venter on both cast and shell with 
very faint tubercles, and the other and the two described next below 
represent the gerontic stage of a shell of very large size of the same 
species, with flattened venter on the cast and very faint concave zone on 
the shell, but no signs of tubercles on this part. The umbilical shoulders 
were destroyed. In both of these specimens from Kansas the ventral 
branches of first lateral saddles did not reach the edges of the venter and 
the same smooth band appeared on either side of this part as in other forms 
of this group. I can not at present separate them from whitfieldi upon the 
basis of their slight tuberculations, since faint tubercles are apt to be 
present at some stage even in typical forms of this species. A fine 
specimen from Nebraska, diameter 228 mm, and covered with beautiful 
nacre, shows chevron markings and has concave ventral zone and faint, 
