2 March, Morphogenesis of certain Pelecypoda. 



This curve had two maxima, one at 68, the other at 

 75, 76, 77 for U. pictonun. On looking up the geo- 

 graphical distribution of the two forms it was apparent 

 that those with a maximum of 76 and 77 were found 

 either on Keuper marls or in districts where, as at Wistow 





CM- ow I" 



Fig. I. 



Thickness : Length curve of U. pidonim. 



Park, near Leicester, the boulder clay was derived in part 

 from the denudation of a Keuper marl district. 



On separating out the Keuper from the non-Keuper 

 forms, two simple curves were got, each with a single 



