412 VENERIDiE. 



occasionally the colouring matter is almost or entirely ab- 

 sent from the general surface, but whenever present is, 

 when closely examined, composed of linear painting, and 

 always exists upon the hinder dorsal area. The diversity 

 of its sculpture is not less striking. This is solely com- 

 posed of concentric costellas, or laminar striae, which vary 

 considerably in thickness, elevation, distance, and, con- 

 sequently, number. In the ordinary or typical form, they 

 are extremely numei-ous, slightly irregular, tolerably strong, 

 rounded above, and very closely disposed. These costellee, 

 in the more elongated and compressed varieties, become 

 converted into sharp lamina?, whose interstices when highly 

 magnified appear radiated with very delicate and scarcely 

 distinct striulae, which are never broad or greatly elevated, 

 and range in approximation to each other from closely dis- 

 posed to moderately distant ; in the former case they are 

 so acute as almost to resemble raised striro, and occasion- 

 ally retain their richer colouring and radiated markings 

 (we have figured one also which is completely destitute of 

 all lineation, except upon the hinder dorsal slope); in the 

 latter case the painting is almost wholly absent. 



The ventral margin is arcuated, particularly in front, 

 where it rises to the dorsal edge in an undisturbed sweep. 

 The front dorsal margin is but moderately incurved and 

 extremely sloping, the hinder one is typically subarcuated, 

 produced, and much declining : these characters in the 

 most aberrant variety are somewhat modified. 



The anterior side is only about half the length of the 

 posterior, and tapers very considerably at its extremity, 

 where it is centrally subangulated. The hinder termination 

 is either subangulated below, or if much attenuated, then 

 bluntly subbiangulated. 



The umbones are prominent, but not swollen ; they in- 



