32 Cinnings —Morphogenesis of Platystrophia. 



d' 



Fig. 17. Group of Platystrophia acutilirata from the base of the acu- 

 tilirata zone (Lower Richmond) Tanner's creek, Indiana, with forms trans- 

 itional to P. laticosta from the same zone, aa' , P. laticosta, after Meek ; 

 bb' , P. laticosta, Tanner's creek, lower Richmond ; cc' , intermediate acutilirat a 

 type; del', very mucronate acutilirata; ee' , normal acutilirata. Author's 

 collection. 



laticosta again makes its appearance. Within the next 20 ft. 

 of strata this laticosta is modified into a typical acutilirata. 

 Fig. 17 will make this clear. The transition has been noted 

 in so many individuals, from so many different localities, that 

 there can be no doubt as to the correctness of this mew of the 

 relationship of these two forms. If any additional evidence 

 were needed, it is furnished by a study of the early stages of 

 acutilirata and by the general angularity and high fold of the 

 latter species ; as well as by the absence of the laticosta type 

 from the Rhynchotrem,a zone, where acutilirata abounds. 



Few individuals of laticosta present pronounced gerontic 

 modifications. Such changes when they do occur produce a 

 shell of extreme gibbosity, and with a large cardinal angle 

 (from 65° to 74° in one specimen), so that the contour of the 

 shell approaches that of P. lynx. Fig. 16, O, illustrates this. 

 Normal growth ceased at stage I : the fold subsequently be- 

 comes lower relative to the size of the shell (cf. stages Zand 

 II), and the frontal profile becomes regularly curved instead of 

 being truncated as in ephebic stages (cf. fig. 16, Cc, with fig. 

 15, e'")\ so that the profile also resembles that of a gerontic 



