26 



( himings — Morphogenesis of Platystrophia. 



present collection two specimens of Platystrophia lynx from 

 vevay, Indiana, both of which have three plications on the fold : 

 One individual has four in theumbonal region ; and the other 

 never developed but three* at any stage. The first has an 

 abnormally low index (1-1) and bears every evidence of lateral 

 cramping during growth. 



This would account for the failure 



12 



Fig. 12. Markedly gerontic individual of Platystrophia lynx from the 

 "lynx beds" of Cincinnati, Ohio. Normal adult growth ceased at varix 

 No. /. Note the very large cardinal angle (125°), extreme gibbosity, and 

 obsolescence of the lateral plications. Dyer collection, Museum of Compara- 

 tive Zoology, Harvard University. 



of the fourth plication. The early growth stages of the second 

 specimen also show an abnormally low index and the failure to 

 develop the full number of plications is therefore probably due 

 to the same cause. The Richmond shells, on the other hand, are 

 very transverse at all stages, especially so in the adult. The 

 correct explanation of the obsolescence of plications in this 

 type, as well as in the laticosta and costata types where it is 

 still more pronounced, is probably to be sought in a readjust- 

 ment of the brachia, producing an elevation and narrowing of 

 the fold. This subject will be more fully discussed in a later 

 paragraph. 



* Both are strictly of the triplicate type. 



