Cuming s — Morphogenesis of Platystrophia. 35 



Fig. 18. Platystrophia acutilirata from Richmond, Indiana, a-a', anterior 

 and ventral views of a very mucronate individual ; b b', an intermediate 

 form ; c-c', narrower, very gibbous form ; d-d" , retrogressive type with stages 

 /and II drawn sepai'ately to the left; e-e", a similar individual showing 

 asymmetrical growth and great increase of the cardinal angle in late growth 

 stages ; /, g, h, i, outlines of other retrogressive individuals showing increase 

 of the cardinal angle in late growth stages ; k-k'" an individual from the 

 base of the acutilirata zone snowing progressive decrease in the cardinal 

 angle. Specimens a-i are from the upper part of the acutilirata zone. 

 Author's collection. 



