178 GENESIS OF THE ARIETID^. 



stage of growth. The lobes and saddles also remained immature, and the 

 pike were in some instances divided. They resemble Cor. latum 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. 



Var. 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 pilee are thicker and more decidedly 

 depressed as they approach the tubercles than in variety B. Wright's figure 

 of Sowerby's original, 1 and his other figures, show affinity with this variety; 

 but it is not practicable to identif} r 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 genie ulse, 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 pilse 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 rotiforme to the adult of kridion, 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 lahcm-like young (Plate III. 

 Fig. 15) of one variety and a variety in which the young are difficult to sep- 

 arate from the adult of kridion by their forms and external characteristics. The 



1 Lias Amm., pi. v. fig. 1-3. 



