ALYPTR^ID^J. 115 



nivea of C. B. Adams. As it is the normal state, the usual rules 

 of priority have been set aside, and C. nivea taken for the name 

 of the species, leaving squama and Lessonii for the principal 

 varieties. The White Slipper is known under all forms (when 

 in good condition), by its shaggy, light-green skin, and by the 

 very peculiar character of the nuclear whirls. These are 

 remarkably small, though the shell is large, standing out from 

 the surface, of a reddish tinge, and crowded with regular trans- 

 verse ribs. The characters have been observed in specimens of 

 all the forms, although the influences which produce Lessonii 

 drawing the shell away from the vertex, generally lead to its 

 abrasion. Sometimes the White Slipper goes to live, when 

 young, into the empty burrow of a boring mussel. In these 

 cases, as soon as it has grown to the width of its cave, it is 

 obliged to develop itself longitudinally, at the same time turning 

 up its sides in the vain attempt to get more room. The corres- 

 ponding slipper limpet of the California coast appears to have a 

 special fancy for this mode of life, as most of the specimens 

 sent have assumed the form now described. It was first found 

 by Mr. Nuttall, and distributed by him a C. exuviata. It was so 

 published in Dr. Jay's catalogue. Dr. Gould, however, figured 

 and described it as C. explanata. It had been previously figured 

 by Valenciennes, in the Voyage de la Venus, as C. perforans, 

 that author supposing that it had made the burrow in which it 

 was found. The designation representing an untruth, it must 

 yield to the latest name, which alone is accompanied by a de- 

 scription. A very singular groove, not found in the Mazatlan 

 specimens, appears in all the specimens of G. explanata, and 

 gives name to the shell. It is, however, a mere accident of 

 growth, differing in every individual, and often not appearing 

 till the animal approaches maturity. A specimen in situ, in the 

 Smithsonian Institution, fortunately reveals the cause of this 

 unique appearance. The creature goes to live in at the outer or 

 pipe-end of the burrow of a bivalve, which remains at the other 

 end after the animal has perished. The growth of the shell is 

 normal till it has attained the breadth of the pipe, be that greater 

 or less. It then increases down the pipe, the vertex of the shell 

 being always turned towards the outer end. There is no groove 

 at this period of its growth, and when the vertex is rubbed off 



