228 PSEUDOCERATITES OF THE CRETACEOUS. 
to have a median marginal division and is bifid, the remaining lobes being 
entire and very small and narrow. The trend of the sutures is about the 
same as in adults, but the bend at the fourth lateral saddle is more abrupt, 
the fourth lateral lobe being a scant one, about one-half the length of the 
third lateral. This proportion begins to alter soon after this, and approxi- 
mates to that of the adult before the end of this volution. The number of 
lobes and saddles is the same; they are simply smaller in size and more 
primitive in outline. A very slight amount of exposure acts upon these 
sutures and alters the outlines, destroying the marginal lobes and saddles. 
At this time, i. e., 7 mm. in diameter, the sutures are somewhat more primi- 
tive on the left than on the right side of the volution, an irregularity that 
subsequently becomes less noticeable. There is probably nothing abnormal 
in this, as it is rare in any animal to find the two sides exactly similar. 
The costee begin very early when the volution is about 7 mm. in diameter 
from lines of involution to venter. 
In the ananeanic substage, when the volution is 4 to 5 mm. in diameter, 
the ventral zone is channeled, as in later stages, and sensibly broader in 
proportion. The form of the sutures, etc., is the same, but the digitations 
are less noticeable. The first lateral saddle on the most perfect side of this 
volution is faintly trifid, all the remaining saddles are entire and club- 
shaped except the smaller ones beyond the fourth, which are now plainly 
seen as mere inflections of the outlines of the magnosellarian saddles. The 
arms of the ventral lobe are single, the first and second laterals are beginning 
to show trifid tops, while the third is broader and more club shaped, and 
shows four nearly equal marginal lobes just beginning to appear. The 
ventral saddle is broad with concave base, the dependent marginal saddles 
being absent. There were nine lobes at this stage, but the nmth on the 
line of involution was very minute. 
The next break exposed an earlier age in the ananeanic substage on 
the first quarter of the same volution. The venter had just become flattened 
and the edges of this zone are faintly crenulated on the cast, owing 
apparently to the presence of extremely fold-like costee which cross the 
venter... These are so indefinite that they were perceived with difficulty. 
The sutures are extremely instructive. The ventral lobe is as deep 
and as broad as the ventral zone; the ventral saddle is very small and 
divided by a siphonal lobe. The sides of the first lateral saddles are 
