6 March, Morpkogdnesis of certain Pelecypoda. 



In laterally wide animals the uplift occurs on the 

 anteriorly-faced part of the flank, where the ornament 

 usually dies away. The diagonals, therefore, do not 

 come into the region of the uplift and so lose this marked 

 bend. It is also absent in any individuals where the orna- 

 ment is not continued to the edge. 



-5 / 



Fig. 6. 

 T. navis. Showing straight diagonals. 



It is clear, therefore, that the deviation from the 

 straight diagonal in the clavellate type of ornament is 

 dependent on the relative growth of the parts of the shell 

 and also on the form of the animal. 



Sojiie Causes of Variation in Form in Pelecypoda. 



As has been shown for the British fresh-water UnionidcB^ 

 the form of the individual is exceedingly sensitive to 

 environment. The length of the shell varies with rate 

 of the current and the consistency of the mud in which 

 the animal lives. The width of the animal varies with 

 the composition of that mud. 



These Unionidce live in very diverse surroundings 

 and are probably a decadent group. These two facts 

 make their range of variation great. Comparing 



* March, 191 1. " Studies in the Morphogenesis of certain Pelecypoda. 

 (i) Prel. Note on the variation of U. pictoruin, etc." Manchester Memoirs ^ 

 vol. 55, No. 8. 



