48 



Cumings —Morphogenesis of Platystrophia. 



It is rather difficult to assign these shells to any one of the 

 common types of Platystrophia. From their occurrence and 

 association, as well as from the aspect of the early ontogenetic 

 stages, it seems almost certain that they are pathologic indi- 

 viduals of the acutilirata group. The individual shown in 

 fig. 25, a, certainly presents every appearance of a normal 

 acutilirata, up to the formation of the first strong growth 

 varix. When normal growth was interrupted the shell re- 

 verted to a simpler, more primitive type, so that the later 

 stages are strongly suggestive of P. lynx. The obsolescence 

 of plications which, we have seen, characterizes gerontic stages 

 only of normal shells, here affects ephebic stages as well, and 

 to a profound degree. In both cases it is the direct result of 

 the failing vitality of the organism. In the former case, how- 

 ever, the failure is a normal phase of ontogeny ; in the latter, 

 it is due to the stress of an adverse environment, to which the 

 organisms attempted, with varying success, to adjust them- 

 selves.* 



25 



Fig. 25. — Pathologic individuals of the acutilirata type from Waynes- 

 ville, O. In a the shell is greatly distorted owing to irregularity of 

 growth, and the plications are obsolescent near the lateral margins. 

 In b the plications become nearly obsolete a short distance from the beak 

 and then continue with nearly normal strength to the margins. Schu- 

 chert collection. 



* This subject has been ably discussed by Hyatt. Genesis of the Tertiary 

 species of Planorbis at Steinheim, Anniversary Mem. Bos. Soc. Nat. Hist. r 

 1880, p. 15. 



[To be continued.] 



