THE SHELLS OF THE GULF OF CALIFORNIA. 199 



It is a remarkable fact in geographical distribution, that the forms 

 imbricatum and rude, which are typical in the west tropical fauna of 

 Central America, reappear, but very sparingly on the Caribbean 

 shores; while C. spinosum, which is far more common, more variable, 

 and more widely distributed, being found (under various names) from 

 California to Chili, has not yet been discovered on the eastern side. 

 Again, the C. radiatum, which is the most delicately formed of the 

 whole group, confines itself to the equatorial western seas, not having 

 been found further north than the Panama district. 



An extremely remarkable specimen of C. spinosum was dredged 

 by Mr. Cuming in comparatively deep water. The net brought up 

 a large stone with a small hole in it, on looking down which Mr. C. 

 perceived a number of spines as though a sea-urchin was lodging 

 there. A blow of the hammer discovered the existence of a large 

 cavity within, communicating with the external world only through 

 this narrow opening. In the hollow of this cavity lay the limpet, 

 turned, as it were, nearly inside out. The creature had gone to live 

 there when young, and being of sedentary habits it did not occur to 

 him that he might be imprisoned for life by his own corpulence, else 

 he would probably have made his escape before he had grown too 

 large. As it was he grew larger and larger, and as the walls of his 

 prison rose up round him on every side, he was obliged to flatten out 

 his shell till it became a plate instead of a cone. At the same time, 

 his body protruding into the hollow, the cup protruded along with it 

 till it stood considerably beyond the shell, of which it was normally 

 an inside partition. Thus our Caljptrssid w r as fixed as immovably 

 as any Pholas, but with this difference in their condition : that the 

 Pholas, being designed for that kind of life, is not troubled with use- 

 less head and eyes, and, moreover, is furnished with two long pipes 

 to convey the water to and from the mouth and gills; while the Cruci- 

 bulum had eyes simply to stare at the wall in the dark, feelers to 

 push the stone, and a long ribbon tongue, armed with hundreds of 

 teeth, to rasp the water. And while encumbered with these unneces- 

 sary appendages, he had not the benefit of water pipes to bring what 

 alone this lock-jawed subject had to feed upon. For this want, 

 however, the economy of the animal provided a remedy. The C. 

 spinocum, in its normal growth, is either spinose or not; the flatter 

 forms being almost always smooth. The spines are developed from 

 prolongations of the mantle, (or thin shell-bearing skin of the animal,) 

 which appear at irregular intervals, though in a regular pattern. 

 Sometimes the whole shell is covered with crowded prickles, (C. his- 

 pidum, Brad.,) sometimes a very few long spines appear at the edge 

 on one side of the otherwise smooth shell. Sometimes the spines are 

 few, large, and hollow, (C. tubiferum, Less.,) each of the outer row 

 communicating through a hole with the inner margin, which is after- 

 wards filled up. Our prisoner worked for his living by constructing 

 very large, long, and open spine pipes, which, instead of standing 

 up at right angles to the shell, were directed back towards the narrow 

 opening in the stone. It would appear that by this means the animal 

 was amply supplied with nourishment, for the shell was above the 

 ordinary size. 



