36 



( ^ii in ings — Morphogenesis of Platystrophia. 



i. e. the individual becomes more and more acuminate through- 

 out its ontogeny. What is true of this individual is true of 

 practically every individual from the same horizon. 



Platystrophia acutilirata presents the most profound 

 gerontic moditications of any member of the genus; and gives 

 rise in even greater degree than P. lynx to what may almost 

 be termed a phylogerontic group of shells representing the 

 paracme of the laticosta line, of which as we have seen 

 acutilirata is but an extreme manifestation. Though P m 



B A . 



Fig. 19. Senile individuals of Platystrophia acutilirata from the upper 

 Richmond of Richmond, Indiana. A, an extremely gibbous and greatly 

 thickened shell ; 1 and 2, median sections showing thickening and reduction of 

 body cavity ; 3, 4, 5, profile, ventral, and posterior views showing pronounced 

 varices 2/, III, TV\ 6 and 7, stages II and I separately drawn. B, another 

 individual ; 1, interior showing profound muscular pit ; 2. stage represented 

 by first growth varix ; 3, ventral valve showing: strong varices ; 4, stage 

 represented by second varix. C, another individual cut transversely in the 

 dorso- ventral plane to show thickening of shell, cr., crura; a., area; mm!., 

 ventral and dorsal muscular platforms ; /, pedicle foramen ; a, normal and b, 

 senile thickening of shell ; d, ventral muscular platform. Author's col- 

 lection. 



