68 CLASSIFICATION. 



4. Abdominal, — with bony gills, and ventral fins behind 

 the thorax. 



5. Branchiostagous, — with gills destitute of bony rays 

 or concealed gills. 



6. Chondropterygious, — with cartilaginous gills, and 

 leathery fins, the common skin being continued over them. 



Cuvier found there was a great deal of difficul- 

 ty and vexation, when an attempt was made to 

 divide them into orders, " established on fixed and 

 precise characters ; but the two great divisions, 

 founded on the character of their bones, as being 

 cartilaginous or osseous, are natural and well mark- 

 ed. The first series, or chondropterygii, have, as 

 a general character, the palatine bones arranged 

 so as to supply the place of those of the upper 

 jaws." He therefore divided them into three or- 

 ders. 



CHONDROPTERYGII. 



1. Cyclostomi, — The jaws fixed in an immovable ring, 

 but the branchial openings numerous. 



2. Selachii, — with branchiae as in the preceding, but not 

 their jaws. 



3. Sturiones, — branchiae opening as usual, in a cleft, 

 protected by an operculum, or gill cover. 



OS SEOUS. 



4. Plectognathi, — maxillary bone, and palatine arch, fix- 

 ed to the cranium. 



5. Lophobranchii, — with complete jaws, but having 

 the branchiae in small (ufts. 



6. Malacopterygii Abdominales, — ventral fins on 

 the hinder part of the abdomen. 



