XI] 



THE SHELLS OF PTEROPODS 



573 



help of which we not only see more clearly the way in which the 

 growth-rate varies from point to point, but we also recognise 

 much better than before, the similar nature of the law which 

 governs this variation in the different species. 



b c d e f g ^ 



X O X 



Fig. 299. Curves obtained by transforming radial ordinates, as in Fig. 298, into 

 vertical equidistant ordinates. 1, Hyalaea trispinosa; 2, Cleodora cuspidata. 



Furthermore, the young shell having become differentiated into a 

 dorsal and a ventral part, marked off from one another by a lateral 

 edge or keel, and the inequality of growth being such as to cause 



Fig. 300. Development of the shell of Hyalaea (Cavolinia) tridentata, Forskal: 

 the earlier stages being the ^^ Pleuroj)us lo7igifilis" of Troschel. (After Tesch.) 



each portion to increase most rapidly in the median line, it follows 

 that the entire shell will appear to have been split into a dorsal 

 and a ventral plate, both connected with, and projecting from. 



