CHARACTERS OF REPTILIA. 445 



nected together, either within the heart or in its immediate 

 neighbourhood, 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 dif- 

 ferent 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, including in some cases even flight, but they 

 are never constructed upon the type of the " wings " of Birds. 

 Lastly, with one or two doubtful exceptions, whilst the ankle- 

 joint is placed between 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 presence of an 

 amnion and allantois in the embryo, the absence of branchiae 

 at all times of life, the possession of only one occipital condyle, 

 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 Crocodile^, and 

 a number of extinct forms ; and with the exception of th 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-carti- 

 laginous, 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. 192). In the 

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

 and the two rami are also anchylosed. In most reptiles, how- 

 ever, the two rami of the lower jaw are only loosely united 

 in the Snakes by ligaments and muscles only, in the Lizards 

 by nbro-cartilage, and in the Crocodilia by a regular suture. 

 In all, the lower jaw articulates with the skull by a quadrate 

 bone (fig. 192, a) ; and as this often projects backwards, the 



