186 PSEUDOCERATITES OF THE CRETACEOUS. 
of outer volution absent, is otherwise perfect and shows many essential facts 
with regard to the affinities of this genus. The last half of outer volution 
or greater part of living chamber is in the gerontic stage. This shows that 
the living chamber was at least one half of a volution inlength. Cragin, who 
had this cast in hand as part of his materials, describes his specimens as being 
‘very sharp in the young [my adult], becoming obtuse along the body- 
chamber of adult [my old age] specimens.” The well preserved part of 
the gerontic venter in this cast is 23 mm. long and for a few millimeters is 
rounded, with two almost imperceptible ridges on the ventro-lateral angles, 
then a hollow occurs as if the shell had been injured, and this is continued 
to the end of the fragment by a very faint channel bordered by faiut ridges. 
The young in the nepionic stage has the usual rounded volution; the 
paraneanic substage was compressed with flat sides, deep involution, and 
broad concave venter, with entire, acute ridges on the borders. This stage 
is, in other words, like the ephebie stage of Protengonoceras, as has been 
already stated from examination of less perfect specimens. This substage 
was introduced gradually by the intervention of a metaneanic substage 
having a helmet-shaped section with a flattened venter and an ananeanic 
substage with transitional characters between this and the nepionic rounded 
volution, but not having the flattened zone on the venter. In the anephebic 
substage the compression and flattening continues to increase, the mvolution 
remains constant, and a very narrow channel still persists on the venter. 
In the full ephebice condition this disappears and the venter becomes 
acute. In the paraephebic substage the venter is subacute, the sides a 
trifle more convex, and volution somewhat broader in proportion to the 
ventro-dorsal diameters, but these changes are slow until in extreme age, 
the paragerontic substage, when, as stated above, the venter becomes 
rounded. I have frequently alluded to the remstatement by degeneration 
at the end of a cycle or in the adult of characters analogous to those of 
younger stages. In this specimen the rounded venter of the paragerontic 
substage has a faint concavity, and this, after what seems to have been 
some injury to the edge of the venter, becomes converted, as described 
above, into a faint channel with ridges on the outer borders. There are no 
marks of tubercles, nor coste, nor folds at any stage so far as these could 
be seen. 
