178 GENESIS OF THE ARIETIDA. 
stage of growth. The lobes and saddles also remained immature, and the 
pile were in some instances divided. They resemble Cor, /atum in the rounded 
outline of the superior and inferior lateral saddles, but are unquestionably roti- 
formian in the equal height of the saddles, and in the prolonged and slender 
marginal lobes. 
Van. C. 
Plate III. Fig. 17. 
This is not the Amm. caprotinus of D’Orbigny, as I supposed in my first review 
of this species, but a local variety of Cor. rotiforme found at Semur. The tubercles 
are very distinct and prominent, and the pile are thicker and more decidedly 
depressed as they approach the tubercles than in variety B. Wright's figure 
of Sowerby’s original,’ and his other figures, show affinity with this variety ; 
but it is not practicable to identify them with other varieties as they are here 
described. 
During the first senile stage, on the second quarter of the tenth whorl, the 
keel and channels may be even more elevated than in the adult, but smooth 
zones appear on either side of the channel ridges. The genicule, whose tuber- 
culated bends are also lower on the sides than in the adult, do not terminate, 
as in that stage, close to the channel ridges, but on the outer edges of the 
smooth intervening zones. 
The abdominal lobe at this period is only one fourth longer than the superior 
laterals, and the inferior laterals are about one tenth shorter than these last. 
There is evidently a tendency, as in other cases of senility, to return to the 
immature proportions of the young. 
In the next senile stage the pile began to lose their prominence and their 
tubercles, and on the last quarter of the tenth whorl the latter had entirely dis- 
appeared. The abdomen also became more prominent on account of these 
changes, and marked alteration in the outline of the whorl occurred still later in 
the more advanced stages of degeneration. The flattening and convergence of 
the sides, however, really began with the advent of the smooth zones, and this 
was the beginning of the metamorphoses which in later stages materially altered 
the outline of the whorl. The keel became more prominent on account of the 
greater shallowness of the channels, though these still retained their lateral 
ridges. A slight increase in the breadth of the sides at this stage would have 
produced a whorl in all respects like Cor. orbiculatum. 
Several collections were closely examined for this purpose, but they did not 
contain many young specimens. We found it difficult to trace the likeness of the 
young roliforme to the adult of sridion, on account of the much greater gibbosity 
of the whorls in the ordinary forms of that species. 
A sufficient number of specimens of rotiforme, however, always show all the 
intermediate stages between the extremely thick /aum-like young (Plate LI. 
Fig. 15) of one variety and a variety in which the young are difficult to sep- 
arate from the adult of sridion by their forms and external characteristics. The 
1 Lias Amm., pl. v. fig. 1-8. 
; 
Di. 
heey 
