618 ZOOLOGY. 
Cuass VIII.—Aves (Birds). 
General Characters of Birds.—We have met in the rep- 
tiles, especially in the fossil forms, many characters indicat- 
ing that birds are by no means so specialized or so well. 
circumscribed a group as was formerly supposed. Such a 
relationship between the two classes has recently been still 
further exhibited by Meyer’s discovery of Archeopteryx mac- 
rura Owen of the Solenhofen slates of the Jurassic beds of. 
Germany, and by Marsh’s discovery of birds with teeth and. 
biconcave vertebra in the Cretaceous rocks of North Ameri- 
ca. On account, therefore, of the close relations between. 
birds and reptiles, Huxley has placed these two classes in a. 
series called Sawropsida, which may be opposed to the Jch- 
thyopsida (Fishes and Batrachians) on the one hand, and. 
the Mammalia on the other, by the following characters :— 
Sauropsida.—There are no mammary glands. There is. 
an amnion and an allantois; the species are oviparous or: 
ovoviviparous, with reproductive organs and digestive canal 
opening into a common cloaca, and Wolffian bodies replaced 
functicnally by permanent kidneys. There is no corpus. 
callosum, nor complete diaphragm. Respiration is effected 
by lungs, never by gills. The heart is three or four cham- 
bered, and there are usually two or three aortic arches; in. 
birds but one ; there are red oval nucleated blood corpuscles. 
The bodies of the vertebre are ossified, but without terminal 
epiphyses. There is a single convex, occipital condyle, in 
connection with an ossified basi-occipital, The ramus of 
the mandible consists of several pieces, the articular one of 
which is connected with the skull by a quadrate bone. The 
ankle-joint is between the proximal and distal divisions of 
the tarsus. The skin usually developes scales or feathers. 
These important characters, derived from Huxley (as are 
many of those given beyond for the class Aves), may remind 
the student of the actual affinities between birds and rep- 
tiles. The former are distinguished from other Sauwropsida@ 
by the following peculiarities :— 
Aves.—The body is covered with feathers, a kind of der- 
mal outgrowth found in no other animals. The fore limbs 
