28 PSEUDOCERATITES OF THE CRETACEOUS. 
the diameter of 5 mm. and is faintly sinuous during the latter part of the 
volution seen in Pl. I, fig. 4. These faint undulations are due to prolonga- 
tions of the coste that pass between the tubercles on the ventro-lateral 
angles. This keel, which is a mere angle on the venter, was obviously 
continuous before the coste appeared and is practically continuous subse- 
quently, owing to the minute character of the inflections. Upon the last of 
the outer volution at this diameter, the first of the fourth volution, the outer 
line of tubercles appears and is connected by coste with the inner line. 
The last are very widely separated at first and become nearer only in later 
stages. The larger costee at this age are separated by single, arcuate costa- 
tions terminated outwardly by minute tubercles and fading out mternally 
about halfway across the volution. The umbilical zones are not differenti- 
ated from the sides. The side is evenly plano-convex from the keel to the 
line of involution and the inner, uncostated part is smooth, except where 
interrupted by the inner tubercles or extensions of the tubercular termina- 
tions of the larger costee. The tubercles and nodes are opposite, not alternate. 
The sutures at this age consist of two large saddles (PI. I, fig. 9), a narrow first 
lateral lobe in the center of the lateral zone and a broad second lateral on the 
umbilical zone. he inflections are all apparently entire. In succeeding 
stages the lobes remain in the same position, there being but one in the 
center of the side between the two rows of tubercles and one on the umbilical 
shoulder, but as the shoulder becomes more prominent and the umbilical 
zone broadens, other lobes and saddles appear internally. The venter 
broadens with age, the keel becoming more obscure in consequence of this; 
the sides become flat and convergent; and the umbilical shoulders broaden 
out more than the venter, the increase in the umbilical zones making the 
umbilici very deep. The keel on the cast at this mature stage is faintly 
undulated, and the lateral costze, although very obscure, cross the venter 
and interrupt the keel between each pair of opposing ventro-lateral nodes. 
These widely separated serrations are so faint on the cast that they entirely 
escaped my observation on former occasions. They in fact can be. felt 
more easily than they can be seen. This is also the case in the earlier 
stages described above. The inner nodes are always larger and fewer 
than those on the edges of the venter. ‘The second lateral saddle remains 
broader than the first lateral, even when the shell is somewhat over 40 mm. 
in diameter. It occupies a trifle more than half of the side and encroaches 
on the umbilical zone (Pl. I, fig. 8). 
