6 March, Morphogenesis of certain Pelccypoda. 



that the hinge line is inclined forwards in the natural 

 position of the shell {i.e., the hinge line, instead of being 

 parallel to the tangent to the ventral surface, vertically 

 below the umbo, would meet it at an acute angle if pro- 



4 years. 



X 1 6 years 



A — space representing the 

 lost wing. 



B = \vin<x. 



X I About lO years 



Fig. 2. 



Diagrams illustrating the loss of the " wing " during grow th stages of 

 A. cygnea. 



duced). The excess of growth posteriorly produces a 

 flattened ventral margin, and removes all appearance of 

 bilateral symmetry from the valves. 



The body, however, keeps more or less parallel to the 

 ventral tangent during movement. This is shown by the 

 fact that the posterior ridge marking the most dorsal part 



