314 



COMPARATIVE ZOOLOGY. 



Fio. -286. Lamprey (1'etromyzon Americanus). 

 lantic. 



At- 



SUBCLASS I. Marsipobranchii. 



The Lampreys and Hag-fish have a persistent noto- 



chord, a cartilaginous 

 skull, no lower jaw, 

 a round, suctorial 

 mouth, horny teeth, 

 one nasal -organ, no 

 scales, limbs, or gill- 

 arches. The gills are 

 pouch -like (whence 



the name of the class), and open separately. They are 



found both in salt and fresh water. 



SUBCLASS II. Pisces Proper. 



The true Fishes have two nasal organs, and well-devel- 

 oped jaws and gill-arches. There are four orders : 



1. Elasmobranchii, having a cartilaginous skeleton, and 

 a skin naked or with placoid scales. The gill-openings are 

 uncovered ; and the mouth is generally under the head. 

 The ventral fins are placed far back; the pectorals are 

 large, in the Rays enormously developed ; and the tail is 

 heterocercal. Such are the Sharks, Kays, and Chimaera, 



FIG. '2S7. Shark (Carcharias vulgarit). Atlantic. 



