GENEALOGY. 63 



The only signs of old age observed in any specimens were the disappear- 

 ance of the genicular and pilee, and increasing flatness and convergence of the 

 sides. 



Coroniceran Series. 



This series is composed of three subseries. 



First Subseries. — The radical form is Cor. kridion, and this species shows direct 

 connection in many varieties with Arn. semicostatum. The similarities of form 

 and characteristics of the young of some varieties in both species indicate mutual 

 affinities, though the young .of other forms of kridion have a very different aspect. 

 Three specimens of Cor. kridion from Mohringen in the Museum of Stuttgardt, 

 named from their arnioceras-like forms Amm. Bodleyi, have the precise character- 

 istics of an immature kridion, viz. divergent sides, an elevated abdomen and keel, 

 and tuberculated pilaB. 1 These grade into the typical Cor. kridion? having in the 

 extremely young stages whorls with gibbous, divergent sides, smooth at first, 

 but becoming more quickly pilated and tuberculated. 



From these forms the transitions are complete to Cor. coronaries, which indeed 

 might be very properly considered as a variety of the same species, since it merely 

 exaggerates all the characteristics of kridion. From this we can pass into the true 

 Cor. rotiforme, 3 in which the adults differ greatly from kridion. The young 4 belong 

 to the typical variety, and are broader and flatter on the abdomen than in kridion, 

 and also have divergent sides and very heavy coarse tubercles. Senility is shown 

 only in specimens of exceedingly large size by the very gradual obsolescence of 

 the tubercles, pilas, and channels, though we have not found any specimens, which 

 had become entirely smooth, or in which the channels had entirely disappeared. 



The transition from Cor. rotiforme to Cor. li/ra 5 is accomplished through a 

 variety, which is separable from the former only by a single character of no 

 special value. The superior lateral saddle is somewhat pointed and narrow, 

 instead of being cut into on the border by numerous marginal lobes, as in roti- 

 forme. The varieties of l/jra 6 which follow have, at an early age, a form similar to 

 that of the adult rotiforme with a more or less elevated abdomen, slightly conver- 

 gent sides, and tuberculated pilaa. There is also a tendency to increase the 

 abdomino-dorsal diameter of the whorls, the pilas becoming more closely set and 

 less prominent than in the first described variety. 7 Senility 8 is indicated by 

 obsolescence of the tubercles and the decreasing width of the abdomen. No 

 specimens were observed in which the pilae had disappeared, though in one 

 specimen they were reduced to broad curved folds, and the channels were almost 

 obsolete, the keel also having been reduced to a low broad ridge. This specimen 

 measured 440 mm., Avhile one at Semur measuring 525 mm. had lost only the 

 tubercles, and the first senile or clinologic stage had but just been entered upon. 



1 If Am. kridioides had been found on the same level with the earliest form of Cor. kridion, it would 

 undoubtedly have to be considered a transitional form between semicostatum and that species; but since it ia 

 not found there, perhaps the safest way is to indicate the descent from semicostatum alone. 



2 PI. iii. fig. 3; Summ. PI. xii. fig. 9. 



* PI. iii. fig. 14-17. 4 PI. iii. fig. 4-9 a. 6 Summ. PI. xii. fig. 13. 



6 PI. iv. fig. 9-14. ' PI. iv. fig. 5, 6. 8 PI. iv. fig. 15, 16. 



