68 GENESIS OF THE ARIETID^E. 



Second S'ubseries. — Some specimens of Amm. stettare, Sow., 1 have a young 

 stage during which the sides become more or less flattened and parallel. These 

 are intermediate between true obtusum and Turneri. The old of the stout va- 

 rieties of oblwsum, and specimens of Turneri in their first senile stage, have char- 

 acteristics similar to those of the adults of stettare, and occasion confusion in the 

 identification of fragments. The extreme senile metamorphoses of Turneri, when 

 the whorl became smooth, the channels shallow, the sides convergent, and the 

 abdomen narrow, occurred, as in other species, at variable ages, sometimes in 

 shells only 102 mm. in diameter. 



The differences between the adult of Turneri and the adult of the next 

 species, Ast. Brooki, 2 are well marked in most specimens, but the stoutest and 

 least involute forms of the latter are very closely allied to the former. The adults 

 of the stouter variety of Brooki retained the channels, the keel remained promi- 

 nent, the sides remarkably flattened, and the pilse in some specimens prominent 

 and like those of Turneri, but the whorls were generally more involute. The 

 young have very close resemblance to the adults of Turneri. The young 3 of 

 Ast. impendens had no very close resemblances to -Turneri at any stage. They 

 repeated the adult characteristics of the stouter variety of Brooki during the 

 nealogic stages, and in the adults exaggerated the normal tendency to conver- 

 gence of the sides, depression of the piloe, and narrowing of the abdomen. The 

 adult of this variety approximated quite closely to the senile stage of stettare. 

 The fine series of figures, given by Wright in his "Lias Ammonites" shows 

 most completely the transitional forms of impendens, and are referred to below 

 in the description of Brooki. Ast. denotatum is simply a more involute form of 

 impendens. 



The next species, Ast. Cottenoti? can be traced directly to the preceding 

 form, and, if my translation of the facts is correct, it is the geratologous offspring 

 of impendens or denotatum. The young 5 were similar to the young and adults of 

 impendens, and also more remotely to the adults of stettare, but the next or first 

 ephebolic stage was precisely similar in all respects, except the sutures, to the 

 first senile stages of impendens. In the adults of one variety this stage retained 

 distinct pilse, though in other specimens the sides became smooth. The involu- 

 tion of the adult whorl was more considerable than in impendens, and the shell 

 closer in this respect to denotatum. 



The extreme variety of Cottenoti had a similar form and development, but 

 was somewhat sharper on the abdomen, and the pilae were wholly confined to the 

 nealogic stages, the adult stage being similar in form and characteristics, except 

 in the sutures, to the extreme old age of impendens. Thus the characteristics 

 of the transient senile stages of Ast. obtusum and other normal species were 

 similar to the permanent characters of the ephebolic and even nealogic stages 

 of degenerate or pathological species like Ast. accelercdum and Ast. Cottenoti. 



1 PI. ix. fig. 2, 3. 2 Summ. PI. xiii. fig. 4. 



3 PI. x. fig. 6-9. * PI. x. fig. "10; Summ. PI. xiii. fig. 5. 



5 PI. ix. fig. 10-11 b. 



