60 PSEUDOCERATITES OF THE CRETACEOUS. 
this large specimen, that it is hardly perceptible. There is, however, a 
broader umbilical zone in the paragerontic substage and a decided umbilical 
shoulder. 
A small cast of this species, No. 19145b, U. 8S. National Museum 
(Pl. III, fig. 14), only 59 mm. in diameter, shows the ephebie stage. The 
diameter of last volution from line of involution to venter on cast is 32 mm., 
the same diameter on opposite part of volution is 22 mm., the wmnbilicus 
being 5 mm. without the shell. The fragments of the shell are not specially 
thick except on the crest of the venter, where it forms a solid keel, and in 
the umbilicus, where it is over a millimeter in thickness. 
This cast is somewhat compressed, but judging from an accompanying 
older specimen it is a medium stout volution with, however, a very acute 
venter. There are two lines of distinct, transversely elongated, radiating 
nodes, about 8 mm. apart, the outer line occupying the centran surface, 
about 7 mm. from the venter. The inner line gradually increases its 
distance from the umbilical shoulder, but on the first part. of the outer 
volution begins to be farther removed from the dorsum than the outer row 
is from the venter. The venter is blunted on the last part of the exposed 
volution and the parephebic substage had probably already begun in this 
specimen. 
The ventral saddle is that of Sphenodiscus, with similar marginal 
saddles at the inner corners, but the aspect of the lobes and saddles is like 
those of older specimens, except that they are shorter and the sutures do 
not overlap anywhere, and do not even approach one another until near 
the umbilical shoulders. The lobes all have flaring tops and the saddles 
phylliform and rather broad bases, flattening out, however, as usual near 
the umbilical shoulders. Nevertheless, on the umbilical zone they are 
again slender and phylliform. 
The first lateral saddles are trifid, narrow at the openings, with a few 
phylliform saddles; the second and third are bifid a trifle longer, but of 
nearly the same form; the third becomes trifid on the oldest part of this 
volution; the fourth is trifid and then becomes quadrifid. These form a 
bent outline, each being a little longer than its neighbor, beginning with 
the first lateral. The fifth is bifid and from thence to the umbilicus there 
is the usual row of entire saddles gradually shortening up and showing 
more and more primitive forms; but they become narrower again on the 
umbilical zone. 
