6 INTRODUCTION. 
GRADE II. AMNIOTA. 
Vertebrates in which there are never fins, never functional gills, 
the respiration being by lungs. In development the embryo becomes 
covered by an embryonic envelope called the amnion, while a second 
_ outgrowth from the hinder end of the digestive tract is concerned in 
the embryonic nutrition and is called the allantois. 
Class I. Sauropsida. 
Body, at least in part, with scales, eggs large. 
Sub-class I. Reptilia. 
Cold-blooded vertebrates, the whole body covered by scales or 
horny plates. The living forms are turtles, lizards, snakes and alli- 
gators (crocodiles) and a New Zealand species Sphenodon. The 
fossil forms are more numerous and include Theromorphs, Plesiosaurs, 
Ichthyosaurs, Dinosaurs, and Pterodactyls. 
Sub-class II. Aves. 
The birds are recognized by their warm blood and their feathers. 
Class II. Mammalia. 
The mammals are as sharply marked by their hair as are the birds 
by their feathers. They have warm blood; except the monotremes 
they bring forth living young which are nourished by milk secreted 
by glands (mamme) in the mother. 
There are a few other terms of convenience which may be defined 
here as they will save much circumlocution. The term Teleostomes is 
applied to ganoids and teleosts, from the fact that they have true jaws. 
The amphibia and the amniotes are frequently united as Tetrapoda, 
from their possessing feet, in contrast to the fishes with fins. 
The geological history of these groups is important; their first 
appearance and their geological range is indicated in the accompanying 
table of the geological periods. 
INTRODUCTORY EMBRYOLOGY. 
The structure of an adult vertebrate can be fully appreciated and the 
bearing of the facts recognized only by a knowledge of the develop- 
ment of the parts concerned. It would often appear, for example, 
that certain organs in different groups were exact equivalents of each 
