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. SELACHTI, with branchiae as in the preceding, but not 

 their jaws. 



3. STURIONES, branchis 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. LOPHOBRAJVCHII, with complete jaws, but having 

 the branchiae in small tufts. 



6. MAL.ACOPTERYGII ABDOMINALES, ventral fins on 

 the hinder part of the abdomen. 



