General Classijication of the Pelecypoda. 



105 



There are two apparent objections to Bernard's con- 

 clusions. The first is Noettling's (17, p. 87), who, in criti- 

 cizing Bernard's statement that the dorsal primary laraellaj 

 of the Heterodonta appear later than the ventral ones, 

 says : — " The view that the dorsal primary lamellffi are older 

 than the ventral ones is . . . supported by the fact that the 

 Bivalves grow in a ventral direction — in other words, the 

 ventral portions of a bivalve shell are younger than the 



Fig. 9. 



LMl 



LP I 



LAI 



LAir 



dorsal ones ; it would be certainly strange if the opposite 

 took place with regard to the hinge, where the ventral part^ 

 would be the older and the dorsal parts the younger ones — 

 tiiat is to say, the hinge would grow just in the opposite 

 direction to the remainder of the shell." This certainly 

 would be strange, but Dr. Noettling overlooks three facts : — 

 Firstly. That, as is shown by the growth-lines, the teeth 

 are formed entirely by secondary thickening which may take 

 place at any point. 



