LAMPREYS AND FISHES OF INDIANA. 157 



KEY TO THE SUB-CLASSES OF FISHES. 



A. Tail heterocercal. 



* Skeleton mostly cartilaginous ; skin naked or covered with 



large bony plates ; mouth placed beneath a long project- 

 ing snout. Chondrostei, ip- 157. 

 ** Skeleton well ossified ; skin covered with hard enameled 

 scales or with cycloid scales ; mouth terminal. 



Holodei, p. 163. 

 AA. Tail outwardly homocercal. 



* All the rays of the dorsal and pectoral fins (except some- 



times the most anterior ray), soft and articulated. Veu- 

 trals, if present, abdominal and without a spine. 



Physostomi, p. 169. 

 ;|c^. Usually with more than one ray of the dorsal, one or more 

 rays of the anal, and the most anterior ray of the ven- 

 trals, spinous. Ventrals, when present, thoracic or jug- 

 ular ; or, if the ventrals are abdominal, then there is a 

 spinous dorsal. Physoclisti. p. 244. 



Sub-class 1. CHONDROSTEI. 



PADDLE-FISHES AND STURGEONS: 



Primitive skeleton mostly cartilagious and uotochordal, . Head and 

 shoulder girdle furnished with membrane bones. Opercular apparatus 

 containing but one or two bones. Snout much produced beyond the 

 mouth. Ventrals abdominal. Tail strongly heterocercal. Skin naked 

 or furnished with bony plates, devoid of true scales. Air-bladder com- 

 municating with the oesophagus by means of a duct. 



This sub class contains two orders. Tts members are the least special- 

 ized of the bony fishes. 

 * Mouth large, overhung by the paddle-like snout. 



Selaehodomi, p. 157. 

 ** Mouth small, sucker like ; snout moderate. Glaniostomi, -^. \b%. 



Order 1. SELACHOSTOMI. 



THE PADDLE-FISHES. 



Maxillary present ; premaxillary absent. Opercular apparatus with 

 only a single small bone. Head produced into an enormous snout. 

 Mouth large, horizontal. Skin naked or with small, stellate, bony plates. 



This order contains but a single family. 



