106 



Mr. M. C. March on the 



Secondly. The growth-lines in tlie umbonal region of the 

 shell show that the earliest formed part of the plateau is 

 due to internal thickening. The first growth-line bends 

 down, then up, cutting the edge of the young shell. If the 

 plateau were formed by downgrowth of the external part the 

 growth-lines would run towards the umbonal region as they 

 do in the underpart of the plateau. If, then, as seems 

 probable from the structure of the shell, the plateau is due 

 to secondary thickening, it cannot be possible to speak of 

 teeth as being dorsal or ventral with regard to each other 



Fig. 10. 



LAET 



LAI 



3b I 

 Development of Cytherea deshayesiana. (After Bernard.) 



when they are formed by thickenings on its upper surface. 

 They can only be more or less internal or external with 

 regard to each other. 



Thirdly. According to Bernard^s hypothesis, the lamellae 

 are derived from radial internal ribs, which, except for inter- 

 calation (which only occurs in later shell-development), 

 remain constant in number and normally develop simul- 

 taneously. Such ribs cannot be regarded as dorsal or ventral 

 with regard to each other. 



