354 REPTILIA. 



These being the common characters of Reptiles and Birds, 

 by which they are collectively distinguished from other Verte- 

 brates, it remains to inquire what are the characters by which 

 they are distinguished from one another. The following, then, 

 are the characters which separate the Reptiles from the Birds : 

 The blood in Reptiles is cold that is to say, slightly warmer 

 than the external medium owing mainly to the fact that the 

 pulmonary and systemic circulations are always directly con- 

 nected together, either within the heart or in its immediate neigJi- 

 bourhood, so that the body is supplied with a mixture of -venous 

 and arterial blood, in place of pure arterial blood alone. The 

 terminations of the bronchi at the surface of the lung are closed, 

 and do not communicate with air-sacs, placed in different parts of 

 the body. When the epidermis develops horny structures, these 

 are in the form of horny plates or scales, and never in the form of 

 feathers. The fore-limbs are formed for -various purposes, includ- 

 ing in some cases even flight, but they are never constructed upon 

 the type of the " wings " of Birds. Lastly, with one or two doubt- 

 ful exceptions, whilst the ankle-joint is placed betiveen the distal 

 and proximal portions of the tarsus, the tarsal and metatarsal 

 bones of the hind-limb are never anchylosed into a single bone. 



These are the leading characters by which Reptiles are dis- 

 tinguished from Birds ; but we must not forget the other dis- 

 tinctive peculiarities in which Reptiles agree with Birds, and 

 differ from other Vertebrates namely, the absence of branchiae 

 at all times of life, the possession of only one occipital con- 

 dyle, and the articulation of the complex lower jaw with the 

 skull by means of a quadrate bone. 



It is now necessary to consider these characteristics of the 

 Reptilia a little more minutely. The class includes the Tor- 

 toises and Turtles, the Snakes, the Lizards, the Crocodiles, and 

 a number of extinct forms ; and with the exception of the Tor- 

 toises and Turtles they are mostly of an elongated cylindrical 

 shape, provided posteriorly with a long tail. The limbs may 

 be altogether absent, as in the Snakes, or quite rudimentary, 

 as in some of the Lizards, but as a general rule both pairs of 

 limbs are present, sometimes in the form of ambulatory legs, 

 sometimes as swimming-paddles, and in some extinct forms 

 modified to subserve an aerial life. The endoskeleton is 

 always well ossified, and is never cartilaginous or semi-cartil- 

 aginous, as in many Fishes and some Amphibians. The skull 

 articulates with the atlas by a single condyle. The lower jaw 

 is complex, each half or ramus being composed of from four to 

 six pieces, united to one another by sutures (fig. 312). In the 

 Tortoises, however, these are anchylosed into a single piece, 



