MAZATLAN UNIVALVES 279 



planata appears exactly to correspond with the Pacific shell, to 

 which it is referred with certainty by DJcr, 9 with unwilling 

 doubt by C. B. Ad. The shell quoted by him from Wee. 

 appears (with hardly a doubt) to be C. incurva. The C. rugosa, 

 Nutt. referred to this species by Jay, agrees in almost all 

 respects ; but differs in the epidermis which though somewhat 

 lamellar is glossy, never shaggy ; and in the young shell which 

 is of a very dark colour, with a Velutina-shaped vertex. These 

 most closely related species are however referred to different 

 subgenera by Messrs. Adams. 



C. onyx begins life like a swollen Planorbis, about '013 

 across, with the apex more or less concealed. It increases as 

 in C. nivea, leaving the vertex free, submarginal, and generally 

 medial, the shell increasing in the plane of the vertex, so that 

 the latter can only be seen when the shell is set on its side. 

 At this period the body of the shell is light horn-coloured, 

 with copious stains of tortoiseshell. As it increases, it develops 

 the lamellose structure and shaggy epidermis of C. nivea, so 

 that the light-tinted specimens of this can with difficulty be 

 separated from the dark specimens of the other, the character- 

 istic vertices being generally lost in the onward progress of the 

 shell. The character of the deck, similar in the young shell, 

 is strikingly different in the adult ; for while it develops the 

 central angle, more or less, the extremities are not sinuated ; 

 and the substance does not display striae of growth, except 

 close to the margin, the principal part being opaque and 

 uniform. In this it closely resembles C. incurva, from some 

 varieties of which it can scarcely be distinguished. The outer 

 surface very rarely develops faint longitudinal undulations, 

 (never sharp ridges as in C. incurva) probably from adher- 

 ing to ribbed shells. Even in specimens with the epidermis 

 very fresh, it is often rubbed smooth at different angles on 

 the back, Pfrom adhering to pebbles rolled by the tide. The 

 inside is richly lustrous, generally brilliantly hepatic, rarely 

 blackish brown, most rarely of a light flesh colour. The 

 smallest (imperfect) specimen is '07 long ; a flat sp. (convex 

 when young) measures long. 1*04, lot. *7, alt. '1. 



A twisted sp. 1'36, '9, '6. 



The largest sp. 2'2, 1'3, '6. 



This shell weighs '66 oz., and displays a thickness of laminae 

 amounting at the umbo to '43 in. 



Hob. Panama; on Strombus Peruvianus and other shells, 

 rare ; C. B. Adams. Mazatlan ; very rare, on shells, &c. 



