XI] 



THE SHELLS OF PTEROPODS 



835 



In the above figures, for instance in that of Cleodora cicspidata, 

 the markings of the shell which represent the successive edges of 

 the lip at former stages of growth furnish us at once with a ''graph" 



'Fig. 412. Cleodora ctispidata. 



of the varying velocities of growth as measured, radially, from the 

 apex. We can reveal more clearly the nature of these variations 

 in the following way, which is simply tantamount to converting our 

 radial into rectangular coordinates. Neglecting curvature (if any) 

 b c d e f g Y 



X O X 



Fig. 413. Curves obtained by transforming radial ordinates, as in Fig. 412, into 

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



of the sides and treating the shell (for simpUcity's sake) as a right 

 cone, we lay off equal angles from the apex 0, along the radii Oa, 

 Ob, etc. If we then plot, as vertical equidistant ordinates, the 



