282 MAZATLAN UNIVALVES 



themselves in the white texture of the shell, sometimes re- 

 appearing at the margin, sometimes altogether 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 

 striae 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 '08 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, becoming 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 laminae, 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 



