466 PHILOSOPHY OF ZOOLOGY. 



creeping upon the rocksunder water in the ]Mauritius, by M. 

 Pekon, which Cuvier referred to the o-enus Onehidium 

 of BccH AX AN, already noticed. We have ventured to in- 

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

 of the first ascertained species. Cuvieb conjectures that 

 it breathes free air, and has accordingly inserted it among 

 the Pulmones aquatiqiie. 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. — Beanchifeea. 

 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 generaonly being met with in fresh 

 water lakes and rivers. The branchiae which constitute 

 their aerating organs, exliibit numerous varieties of form, 

 position, and protection, and furnish valuable characters 

 for their methodical distribution. 



Order I. — Braxchi.e exteknal. 



The branchiae are pedunculated, and more or less plu- 

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

 in g^eneral, 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 dorsallv. 



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 hciiTiaphrodite, with reciprocal impregnation. 



