314 
COMPARATIVE ZOOLOGY. 
Subclass I.— Marsipobranchii. 
The Lampreys and Hag-fish have a persistent noto¬ 
chord, a cartilaginous 
skull, no ldwer 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. 
Fio. 2S6.—Lamprey (Petromyzon Aviericanw). At¬ 
lantic. 
# 
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 Eays enormously developed; and the tail is 
heterocercal. Such are the Sharks, Eays, and Chimaera. 
Fig. 2S7.—Shark (Carcharias vulgaris). Atlantic. 
