70 INTRODUCTION TO CLASSIFICATION. 



The blood is cold, and the majority of the blood-corpuscles are' 

 red, oval, and nucleated. The bronchial tubes are not connected 

 at the surface of the lungs with terminal saccular dilatations, 

 or air-sacs. When, as is ordinarily the case, the superficial 

 layers of the epidermis of Keptiles are converted into horn, the 

 corneous matter takes the form of broad plates, or of overlapping 

 scales, neither plates nor scales being developed within pouches 

 of the integument. 



XXVII. THE AVES. 



The class of Birds consists of animals so essentially similar 

 to Reptiles in all the most essential features of their organiza- 

 tion, that these animals may be said to be merely an extremely 

 modified and aberrant Reptilian type. 



As I have already stated, they possess an amnion and a respi- 

 ratory allantois, and the visceral arches never develop branchial 

 appendages. The skull is articulated with the vertebral column 

 by a single condyle, into which the ossified basi-occipital enters 

 largely. Each ramus of the lower jaw, composed, as in 

 Reptiles, of a number of pieces, is connected with the skull 

 by an os quadratum, to which the hyoidean apparatus is not 

 suspended. 



In no existing bird does the terminal division of the 

 fore-limb possess more than two digits terminated by claws, 

 and the rnetacarpal bones are commonly ankylosed together, 

 so that the "manus" is of little use, save as a support for 

 feathers. 



In the hind limb of all birds the distal tarsal bone and the 

 metatarsal bones become more or less completely ankylosed 

 together, so as to form a single osseous mass, the " tarso-meta- 

 tarsus." 



The greater and lesser circulations of birds are completely 

 separate, and there is only one aortic arch, the right. The right 

 ventricle has a muscular valve. The blood is hot, hotter on the 

 average than that of any other vertebrates, and the majority of 

 the blood-corpuscles are oval, red, and nucleated. The bron- 

 chial tubes open upon the surface of the lungs into air-sacs, 



