Part III. 
SYSTEMATIC SYNOPSIS 
OF 
NORTH AMERICAN BIRDS. 
CLASS AVES: BIRDS. 
THIS CLASS OF ANIMALS, while sharply distinguished from Mammals, is so closely 
related to Reptiles, that the presence of feathers in the former, and their absence 
from the latter, is the most obvious if not the only positive character by which the two classes 
are separable. 
Though the species of birds are numerous (some 10,000 are known), the structural diver- 
sity of the Class is comparatively so slight, that the characters upon which the primary divisions 
are based seem insignificant in view of those upon which the major groups of Mammals or 
Reptiles may be founded. With strict regard for equivalency of taxonomic groups, based on 
moi-phological considerations, the conventional class" of Birds is scarcely or not of higher 
value than an order of Reptiles, with which Birds are associated under the name Saurop- 
SIDA. But it is not proven that a given structural character may not have classificatory value 
in one case, different from that which may properly be attributed to it in another ; so that, 
though the most diverse birds may be more alike than are extremes among Lizards for 
example, we may still continue to speak of a class Aves, to be primarily divided into sub-classes 
or orders. 
All known Birds, living and extinct, are divisible into the following primary groups, 
which may be termed sub-classes : 
I. Saurur^. — Birds with teeth. Vertebrae biconcave (amphicoelous). Sternum 
keeled. Wings small, with separate metacarpals. Tail longer than body, its 
vertebrae not pygostyled, its feathers arranged in distichous series. (One species, 
Af'chceopteryx lithographica, from the Jurassic of Europe. Fig. 14.) 
II. Oooi^TOTORMiE. — Birds with teeth, implanted in sockets. Vertebrae biconcave. 
Wings large, with anchylosed metacarpals. Sternum keeled. Tail short. 
(Typified by the genus Ichthyornis, from the Cretaceous of North America, 
ifig. 16.) 
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