188 EEPTILIA. 



the surface of the lung arc closed., and do not communicate with 

 air-sacs, placed in different parts of the hody. Wlien the epi- 

 dermis develops horny structures, these are in the form of horny 

 plates or scales, and never in the form of feathers. The fore- 

 linibs are formed for varioits purposes, including in some cases 

 even fight, hut 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 hetween the distal and proximal 

 portions of the tarsals, the tarsal and metatarsal hones of the 

 hind-limb are never ancliylosed into a single hone. 



These are the leading characters by which Eeptiles are 

 distinguished from Birds ; but we must not forget the other 

 distinctive peculiarities in which Eeptiles agree with Birds, 

 and differ from other Vertebrates — namely, 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 

 Tortoises and Turtles, the Snakes, the Lizards, the Crocodiles, 

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

 the Tortoises 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 cartilagi- 

 nous or semi-cartilaginous, as in many Fishes and some Am- 

 phibians. 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 an- 

 other by sutures (fig. 546). In the Tortoises, however, these 

 are anchylosed into a single piece, and the two rami are 

 also anchylosed. In most Eeptiles, however, the two rami of 

 the lower jaw are only loosely united — in the Snakes by 

 ligaments and muscles only, in the Lizards by fibro-cartilage, 

 and in the Crocodilia by a regular suture. In all, the lower 



