CLASS II. PISCES. 



THE FISHES. 



Skeleton more or less ossified; skull separate from the segmented 

 vertebral column; mouth with true jaws; limbs, shoulder girdle, 

 and pelvic bones usually present ; gills attached to imperfect skeletal 

 arches, usually less than 6 gill openings on each side; nostrils one or 

 more pairs; heart with arterial bulb; alimentary canal variable in 

 form, and with pancreas and spleen. 



KEY TO THE ORDERS OF PISCES. 



a. Tail heterocercal ; vertebras opisthoccelian (con- 'PAGE 

 cavo-convex) ; air bladder cellular ; scales rhom- 

 bic, enameled plates Rhomboganoidea 4 



aa. Tail not heterocercal; vertebrae amphiccelian 



(double concave); air bladder, if present, not 



cellular; scales, if present, of the ordinary sort. 



b. Anterior vertebras (about 4) much modified, 



co-ossified, and provided with ossicula audi- 



tus; ventral fins, if present, abdominal and 



without spines. 



c. Maxillary bone imperfect, forming the base 

 of a conspicuous barbel ; no subopercle ; no 



scales Nematognathi 8 



cc. Maxillary bone perfect (rarely wanting), 

 never entering into the base of a barbel; 

 subopercle present; scales usually present. 



Plectospondylii 24 



bb. Anterior vertebrae unmodified, similar to the 

 others or more elongate, separate and with- 

 out ossicula auditus. 



d. Body eel-shaped; vertebras numerous (100 

 to 250); scales minute or wanting; no ven- 

 tral fins ; pectorals usually present ; gill open- 

 ings restricted ; four pairs of gill arches. 



e. Premaxillary, maxillary, and palatine bones 

 well developed and distinct from each 

 other as in ordinary fishes; pectoral and 

 ventral fins wanting; gill openings con- 

 fluent Symbranchia 89 



