1 Introduction. 



system, as observed in existing Birds, and the fossil remains of 

 sucli transitional forms as are preserved in Archaeopteryx on the 

 one side, and the Dinosaur'ia on the other, show that their more 

 essential morphological affinities are distinctly Reptilian. The 

 aberrant integument of Birds, being, as it is, by virtue of its 

 polished surface, an imperfect radiator, and, by virtue of the layers 

 of air it entangles, an exceedingly bad conductor of heat, is a 

 powerful auxiliary in the economization of the heat generated by 

 the rapid rate at which their various functions are carried on. Both 

 Rei^tiles and Birds are favourably conditioned for the conservation 

 of heat, by the semi-solid character of their excreta ; and at par- 

 ticular seasons, as has been observed in the case of the incubation 

 of the Python, Reptiles do appear to obtain the power of raising 

 their temperature considerably above that of the medium in which 

 they live. The skeleton of Birds contrasts with those of Reptiles 

 and Mammals generally, by its greater hardness and lightness, 

 and its greater readiness to form anchyloses. The cervical and 

 dorsal vertebrae have their centra articulated by synovial joints, in 

 which cartilaginous menisci are to be found. The anterior surfaces 

 of these centra have the procoelous appearance when looked at 

 in situ and from in front; but when one of these vertebrae is 

 removed from apposition with the one next in front of it, the 

 anterior surface of its centrum is seen to be saddle-shaped or cylin- 

 droidal transversely, whilst the posterior surface, being conformed 

 so as to articulate with an anterior surface of that shape, is, in 

 its turn, convex transversely, but concave from before backwards. 

 On the other hand, the occipital condyle, at least of the more typical 

 Birds, is more perfectly spheroidal as retaining less trace of its 

 trifid comj)Osition out of the basi-occipital and the two ex-occipitals 

 than in most Reptiles. The neck vertebrae may vary in number 

 from nine to twenty-four, the dorsal from six to ten, of which the 

 four or five most anteriorly placed are ordinarily anchylosed with 

 each other, except where, as in the Hatitae and some of the Cari- 

 natae, as the Penguin, the power of flight is lost. The sacral 

 vertebrae vary in number from nine to twenty, the enormously 

 elongated ilia abutting directly upon them without the interposition 

 of any sacral ribs as in Reptiles, or the separate centres of ossifi- 

 cation which represent those ribs in Mammals. There are from 

 eig-ht to ten caudal vertebrae, the last of which forms an ' os en 



