204 LECTURES ON 



it is obliged to develop itself longitudinally, at the same time turn- 

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

 sponding slipper limpet of the Californian 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 as 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 repre- 

 senting an untruth, it must yield to the latest name, which alone is 

 accompanied by a description.* A very singular groove, not found 

 in the Mazatlan specimens, appears in all the specimens of C. expla- 

 nata, 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 specimeu, in situ, in the Smith- 

 sonian Institution fortunately reveals the cause of this unique appear- 

 ance. The creature goes to live at the outer or pipe-end of the 

 burrow of a bivalve, f which remains at the other end after the ani- 

 mal 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, (as it generally is in elon- 

 gated specimens,) it can hardly be distinguished from similar speci- 

 mens of the White Slipper. But as soon as it has reached the bottom 

 of the pipe, where the dead bivalve (generally a Petricola, a creature 

 with rather short siphons) still remains undecomposed, it suddenly 

 encounters an unexpected obstacle. It wedges itself under this (to 

 it) mighty globe, and turns its delicate mantle, exuding the shelly 

 skin, up the sides of the new-moon-shaped cavity, but in vain. There 

 is nothing for it but to retrace its steps, and back out. As it does 

 so, every new portion, formed under the arched bivalve, repeats the 

 previous concave impression, and the Grooved Slipper is the result. 

 The sharp instrument of the •explanation" of one author, and the 

 " perforation" of the other, is nothing but the little rounded " clam," 

 tightly wedged at the bottom of its burrow; and the same slipper- 

 limpet, freely developed under unconstrained influences, is probably 

 the C. navicelloides of Nuttall, to ascertain the characters of which 

 we are still in want of perfect specimens. 



To return to the White Slippers on the back of our Thorn-Oyster. 

 Among the young shells which appeared to the naked eye to be the 



are essentially distinct. Man is not a monkey, although certain unhappy idiots may 

 appear less highly organized than the lower order. 



~ It is greatly to he regretted that in this country, where type series, named by Mr. 

 Nuttall himself, were readily accessible, his labors should have been so often disregarded. 

 On the other side of the Atlantic they have frequently found their way into the mono- 

 graphs, but unfortunately too late for preservation. 



f These burrows will be found described at page 209, et seq. 



