434 T. Gill—Number of Classes of Vertebrates. 
Elasmobranchiates. : 
2. In the second (Quadratifera), the shoulder-girdle is repre- — 
sented by the scapulz and their appendages, which are limited 
to the respective sides, a sternum is differentiated, and instead 
of an air bladder are two lungs, each with a special canal, which 
communicate with the pharynx. The lower jaw is compoun 
and is articulated with the skull by the intervention of a special 
bone—the os quadratum. In this superclass belong the Ba- 
trachians, the Reptiles, and the Birds; the last two forming the 
group Sauropsida. / 
8. In the third (Malleifera), the shoulder-girdle is represented 
by composite scapule, limited to the sides or back; a pn 
developed; respiration is entirely effected by highly specializ 
superclass is represented by a Se a prance ft the 
ied in a el 
i true Fishes and 
As to other questions, that is, whether the be wal Ot 
sistently withheld from the Fishes and Elasmobranchiates ery 
I have heretofore hesitated, it is because of Dr. Giinther's ¥ 
verse views. ’ ‘on as 
Without prejudice to the reconsideration of the qo itt 
to the systematic value of the group of Selachians OF ‘buted, 
branchiates, the classes of Vertebrates may then be distribu®"» 
m a descending series, as follows :— 
