92 PSEUDOCERATITES OF THE CRETACEOUS. 
and an alternating shorter one. This last series disappears in the last part 
of the outer volution of the specimen described, leaving a series of single 
costs. These may perhaps be obsolete or obsolescent in older specimens. 
There is a decided aspect of bifurcation in the earliest stage observed when 
from line of involution to venter the cast of the volution is 15 to 20 mm., 
but there are no tubercles. The whole diameter of this cast is 67 mm. 
The outer volution at largest end from line of involution to venter is 37 mm.; 
without the shell the umbilicus is 7 mm. on one side and 8 mm. on the 
other side; with the shell it would be 5 mm. on one side and 6 mm. on the 
other. The greatest transverse diameter is about one-third of lateral zone 
distant from umbilicus and is for thickest part 18 mm., and opposite, cor- 
responding to ventro-dorsal diameter above given, it is 11 mm., both 
measurements being between the costee. The costz are more prominent on 
the middle and inner parts of the sides than on the outer, and broaden 
out toward the venter, disappearing near the solid edge somewhat abruptly 
on the younger parts and more gradually along the older part of this 
volution. 
The first lateral saddles are broad and short trifid on the right side, 
with a small outer marginal saddle and quadrifid on the side figured (Pl. 
X, fig. 7), with an inner accessory large marginal saddle dependent on the 
first laterals. The second laterals are single, short, but slender saddles, 
with phylliform bases, which are only slightly indented at the age observed, 
The third laterals are only about a third longer than these, and, although 
overlapping, are easily separated. The outlines are irregular and deeply 
indented by two marginal lobes and other accessory lobes. The fourth and 
fifth laterals are phylliform, with the bases indented by a number of 
marginal lobes, apparently entire and small. The sixth and seventh laterals 
are faintly trifid. The almost entire aspect of these saddles is due to slight 
abrasion. On the youngest part of this volution (PI. X, fig. 9) all of the 
saddles are entire except first to third, the cast here being 16.5 mm. in 
breadth from line of involution to venter; the outlines are perfect. 
The ventral saddles are more like those of Sphenodiscuss than in 
novimexicanum, and have rounded marginal saddles at the inner angles. 
The first lateral lobes are of about the same length as the arms of the 
ventral lobe, both of them trifid and with accessory marginal lobes orad of 
these. The second laterals are the deepest lobes, and represent the primitive 
