THE SHELLS OF THE GULF OF CALIFORNIA. 207 



This singular structure, to which there is some approach in the 

 fossil genus Nerinrea, ran along most of the whirls, becoming evan- 

 escent in the earlier and later portions. On examining similar shells 

 from other seas, I found species in all the principal zoological 

 provinces, each characterized by a different growth of the internal 

 laminse. They had escaped observation before. I presume, because 

 of the love entertained by collectors for "perfect" shells. Mr. 

 Woodward, the author of the invaluable "Manual of the Mollusca," 

 (Weale & Co., London; 3 parts,) enabled me to affiliate them to a 

 genus established by Dr. Lea, (under the name Petaloconchus,) for a 

 tertiary fossil of the United States, in which, however, the peculiar 

 character is but slightly developed. 



If time allowed we might dwell on a number of other interesting 

 shells which were found either living on the backs of the Thorn-oyster 

 shells, or accidentally lodged between the foliations of the valves. 

 We must confine our attention to a few. Among them were eleven 

 species of Chitonidas or Woodlouse shells, of which eight were new. 

 This strange family of Mollusks, while agreeing in many essential 

 particulars with the true limpets, present some curious points of 

 analogy with the articulated animals, having their skin -skeleton 

 broken up into joints, and exhibiting a symmetrical and double arrange- 

 ment of the organs of the body. Among them was one specimen of 

 extraordinary beauty, though not much more than the tenth of an 

 inch in length. Under the microscope each of the valves displayed 

 a very elaborately ornamental sculpture, richly tinged with green, 

 purple, pink, and brown; their shape, with the pointed beak and the 

 transparent wing like processes at the side, bore no very fanciful 

 resemblance to a bird in flight; while the thick skin in which they 

 are imbedded was covered with minute transparent prisms, and at 

 regular intervals with what the microscope revealed as clusters of 

 white crystals, glittering in the reflected sunlight like the finest 

 specimens of arragonites. The same species has just been sent to 

 the Smithsonian Institution, adhering to similar shells from Cape St. 

 Lucas, by that indefatigable collector, Mr. Xanthus. 



Sheltered from rough usage between two layers of shell Was a new 

 form of Isognomon, which may be called, in English, the " shoulder- 

 of-mutton shell with the double face ;" having one of its valves 

 smooth, while the other has beautiful radiating lines covered with 

 imbricated scales. These creatures, along with the Pinna shells, 

 (which, like the Spondyli, have preserved the same name since the 

 days of Aristotle,) the Hammers, and the Pearl-oysters, moor them- 

 selves to fixed objects by a byssus or anchor cable of their own spinning. 

 Whenever the most minute fragment of shell belonging to any species 

 of this family is examined under the microscope it always presents 

 a prismatic cellular structure, like basaltic rocks in miniature, or like 

 the "float" of a belemnite. When a large old Pinna has long been 

 exposed to the action of the weather, its surface will crumble into 

 these prisms in the hand. They are formed by the breaking down of 

 a longitudinal row of ordinary cells, like the ducts of plants. It appears 

 that all shells are originally formed by the aggregation of cells in the 



