FOSSIL VERTEBRATES — AMPHIBIA 505 
four original pieces are still separate. This is the stereospon- 
dylus condition (Fig. 2). 
Another line of development that is quite closely connected 
with the geological history of the animals is the development of 
2 
Vv vy. atl 
A B 
Fic. 2.—A Rachitomous vertebra. 4 Embolomerous vertebra. a@ Intercentrum; 
4 Pleurocentrum; c Neural spine. 
the shoulder girdle. In the more primitive forms, especially the 
Lranchiosauria of the Permian, the bones of the shoulder region 
were largely cartilaginous and were small in size; as we pro- 
gress through the Carboniferous and into the Triassic time the 
bones become larger and firmer, until in the latest of the Trias- 
sic forms they form a close dermal armor over the underside of 
the thoracic cavity. In the majority of the forms there is devel- 
oped in the skin of the abdomen and the lower side of the legs 
as well, a large number of small bony ossicles that served asa 
protection for the posterior part of the body. The history of 
these is also closely connected with the geological progress of 
the animal; in the earliest forms the ossicles are small and 
numerous; as the forms progressed the ossicles became larger 
and less numerous. 
BRANCHIOSAURIA. 
This is the most primitive group of the 
Amphibia. The forms were all small and salamandriform; there 
was a short tail, and the limbs were short and weak. The ver- 
tebree in the adult form were barrel-shaped as already described 
and in the immature form composed of two separate pieces, an 
upper and a lower. There was no plication of the enamel of 
