208 WEST COAST SHELLS 



perched upon the shell of the black turban, T'cgula 

 funebrale. 



Crepidula onyx, Sby., the On}'x Slipper-shell, 

 somewhat resembles the last species, but in favorable 

 locations it grows to a much greater size. The sur- 

 face is somewhat roughened or shaggy, and the apex 

 is on the very edge of the shell. Beautiful specimens 

 are found in San Pedro Bay, some of them measuring 

 fully two inches in length and an inch and a half in 

 breadth. The inside is a rich, dark brown, very 

 glossy, while the deck is as white as the purest mar- 

 ble. There is another name, C. rugosa, Nutt., which 

 is applied by some to these, or similar specimens, 

 while by others rugosa is considered as a variety of 

 onyx. Carpenter believed rugosa to be the northern 

 form of the Mexican species, Crepidula onyx. 



Crepidula nivea, Gould, (C. navicel- 

 loides, Nutt.), the White Slipper-shell, 

 Figure 202, may easily be recognized by 

 its color, its flattened shape, and by the 

 thin and delicate deck, which is shown in 

 the engraving. Sometimes this mollusk 

 makes its home upon the rock and the 

 back of his shell becomes rough and discolored; 

 while again living specimens may be found within 

 the aperture of a dead spiral shell, and then the 

 slipper is smooth, curved, elongated, and almost 

 transparent. This species is exceedingly variable, 

 and numerous names have been given to it and its 

 varieties. Its shell conforms to circumstances in a 

 remarkable manner, being straight and narrow if it 

 lives on such a shell as that of Glottidea albida. 



