466 PHILOSOPHY OF ZOOLOGY. 



creeping upon therocksunder water in the Mauritius, by M. 

 PERON, which CUVIER referred to the genus Onchidium 

 of BUCHANAN, already noticed. We have ventured to in- 

 stitute the genus, and to name in honour of the discoverer 

 of the first ascertained species. CUVIER conjectures that 

 it breathes free air, and has accordingly inserted it among 

 the Pulmones aquatique. Some doubts, however, may 

 reasonably be entertained as to the truth of this supposi- 

 tion. It would certainly be an unexpected occurrence, to 

 find a marine gasteropodous mollusca obliged to come to 

 the surface, at intervals, to respire. It will probably be 

 found that it is truly branchiferous. 



Class II. BRANCHIFERA. 



The molluscous animals of this class are more numerous 

 than those of the preceding. They chiefly inhabit the wa- 

 ters of the ocean, a few genera'only being met with in fresh 

 water lakes and rivers. The branchiae which constitute 

 their aerating organs, exhibit numerous varieties of form, 

 position, and protection, and furnish valuable characters 

 for their methodical distribution. 



Order I. BRANCHLE EXTERNAL. 



The branchiae are pedunculated, and more or less plu- 

 mose. They are moveable at the will of the animal, and, 

 in general, are capable of great alteration of form. 



1st Tribe. 



Branchiae exposed. In nearly all the genera, the bran- 

 chiae are numerous and distributed regularly over the cloaks 

 or sides. 



A. Branchiae issuing from the cloak dorsally. 



1. Body exposed, and destitute of a shell 



In many species the back is covered with perforated pa- 

 pillae, which pour out a mucous secretion. All the species 

 are hermaphrodite, with reciprocal impregnation. 



