282, MAZATLAN UNIVALVES 
themselves in the white texture of the shell, sometimes re- 
appearing at the margin, sometimes affogether absent. Very 
rarely a rich lustrous brown is developed inside, as in C. onyx, 
shading into a sea-weed green. The shell is covered with a 
- copious yellowish-green epidermis, which lies in a fringe round 
the sharp margin. Under this, the surface presents concentric 
striee of growth, and very fine longitudinal corrugations. Either 
of these however may disappear, or be developed at the ex- 
pense of the other. The vertex is either in the margin, or as 
much as ‘O8 removed from it; it is not imbedded in the sur- 
face of the shell as in C. unguiformis, but is slightly prominent, 
displaying its rugose folds. Both these and the vertex itself 
are often rubbed off. 
In its ordinary state, the texture has a tendency to run into 
layers. The epidermis does the same, the layers being in 
shreds and very copious. These layers in the adult are often 
continued to the margin, making it broad and flat. At other 
times they stand out more or less on the back of the shell. 
Sometimes the shell advances longitudinally, making layers at 
more or less regular intervals, beeoming then the aberrant 
form C. Lessonii; which is however connected by such gradual 
links, through C. striolata, with the typical C. nivea, as not to 
admit of separation. In this form, (common in some places, 
but extremely rare at Mazatlan, the vertex is generally left 
behind and rubbed off: in the only S. American specimen in 
which it was found, it exactly agreed with the remarkable 
character of C. nivea. The specimens in Mus. Cuming from 
Vancouver’s Str. (without authority.) are large and spreading ; 
with the apex small and ribbed as in C. nivea, but white. 
Of the shells collected by Mr. Nuttall at Monterey &c., some 
want the vertex, but otherwise resemble C. nivea: one very 
young specimen however has a specifically different vertex, 
more resembling Cr. fornicata, &c. The C. explanata of Gould 
is probably a variety of the Monterey shell, caused by living 
in the hole of a Lithophagus ; similar forms of C. nivea being 
found among the Mazatlan shells. It differs in the cancellated 
structure between the lamin, which however is not seen in 
the Cumingian specimens. In its young state, the shape is 
normal. If the species should prove distinct, the name of 
Gould must take the place of the prior names of Nutt. and 
Val., the former not having been described, and the latter 
representing an untruth. 
The growth of C. nivea is rather straight, after the nuclear 
portion is completed, with the vertex submedial or directed 
