MAZATLAN UNIVALVES 279 
planata appears exactly to correspond with the Pacific shell, to 
which it is referred with certainty by Dkr,, with unwilling 
doubt by C. B. Ad. The shell quoted by him from Mke. 
appears (with hardly a doubt) to be C. mcurva. The C. rugosa, 
Jutt. 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 ight 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 strize of growth, except 
close to the margin, the principal part bemg 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, ?from 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 bongs \.04, tat. °7, aléiNs 
A twisted sp. 4 . (Cm Om eta? ey OM Li. 
The largest sp. DBs). cee cl ae. | oeaatan 
This shell weighs °66 oz., and displays a thickness of lamine 
amounting at the umbo to °43 in. 
Hab.—Panama; on Strombus Peruvianus and other shells, 
rare; C, B. Adams,—Mazatlan; very rare, on shells, &c. 
