518 ZOOLOGY. 



CLASS VI. 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 Archmopteryx litho- 

 graphica of the Solenhofen slates of the Jurassic beds of 

 Germany, and by Marsh's discovery of birds with teeth and 

 biconcave vertebrae in the Cretaceous rocks of JSForth Ameri- 

 ca. On account, therefore, of the close relations between 

 birds and reptiles, Huxley has placed these two classes in a 

 series called Sauropsida, which may be opposed to the Icli- 

 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 caual 

 opening into a common cloaca, and "Wolffian bodies replaced 

 functionally by permanent kidneys. There is no corpus 

 callosum, nor complete diaphragm. Respiration is effected 

 by lungs, never by gills. The heart is two or three 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 vertebras 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 Sauropsida 

 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 



