GASTEROPODA. 199 



Class GASTEROPODA. 



(See Vol. I. p. 51.) 



In considering the natural distribution of this group, it 

 will be found that the systems of classification which 

 have been propounded by naturalists since the post- 

 Linnean revolution are so numerous, that the student 

 is apt to be lost in the perplexing labyrinth into which 

 they lead him. That of the great Cuvier, however, 

 seems to have stood its ground better than any other, 

 and is commended by its greater simplicity. It is 

 founded on differences in the nature and position of the 

 gills or respiratory organs. Some modification has 

 been rendered necessary by the investigations of later 

 physiologists ; and I will submit a scheme, which 

 appears to me sufficient to classify the Gasteropoda, 

 without making any pretence to novelty or perfection. 

 I would adopt the following eight orders. 



1. Cyclobranchiata, (Cyclobranches) Cuvier. 



Gills arranged in two separate rows, one on each side of 

 the body, and covered by the mantle. Chitonidce. 



2. Pectinibranchiata, (Pectinibranches) Cuvier. 



Gills consisting of one or two plumes (usually a single 

 plume), placed above the head, or on either side of it, and 

 covered by the mantle. Patellidce, Trochidce, and many other 

 families, having (invariably in the young state) a spiral or 

 turbinated shell with an entire mouth. 



3. SlPHONOBRANCHIATA, GoldftlSS. 



Gills consisting of one or two plumes, placed obliquely on 

 the anterior part of the back, and contained in a cavity of the 

 mantle, which is prolonged into a tubular canal. Mv.ricidce, 



