REPTILES (Lat. repo, I creep), constitute a class of the subkingdom vertebrata, lying between the classes of amphibians and birds. They may be briefly characterized as being cold-blooded, having a heart composed of only three cavities—viz., two auricles and a single ventricle, and as breathing by lungs throughout the whole period of their existence; in which respect they differ from the amphibians, which some zoologists asso ciate with them, and which, in the early part of their existence, are furnished with gills for aquatic respiration. They are divided into the following orders: 1. Ophidia, or ser pents; 2. Sauria, or lizards: 3. Loricala, or crocodiles; and 4. Chelonia, or tortoises; so that in so far as external .form is concerned, the members of this class present a far greater diversity than is observed amongst the members of the other classes of verte brates.
With the exception of the tortoises, the reptiles in general are of an elongated form, the body being often nearly cylindrical, and usually terminating in a very long tail. In a considerable number (as the serpents and some of the lizards) no traces of limbs are apparent; in some (as certain lizards), the limbs are rudimentary; while in the remainder the limbs are fully developed, although not to the extent to which development takes place in birds or quadrupeds,•as the feet rarely suffice to keep the belly from the ground. The outer covering of the body presents several well-marked varieties. In a few of the lizards, the skin is covered with regular scales, composed of a mixture of bony and horny matter, and lying over each other like those of fishes; in most lizards and in ser pents, there are scales and plates developed on the surface of the corium or true skin, and covered over with epidermis, which is thrown off at intervals, the moult forming au accurate• cast of the body of the animal; while hi the crocodiles and tortoises the scales are converted into true bony plates, which in the former are imbedded in the tissue of the skin, and in the latter are united with the ribs, sternum, etc., of the internal skeleton, to form the complete bony case into which the head and limbs of the animal can be retracted.
The skeleton is completely ossified in all reptiles, and presents many points of interest to the philosophical anatomist, into which we have not space to enter. In the skeleton of the crocodiles and lizards, there is an obvious distinction of the regions of the neck, trunk, and tail. The total number of vertebra; is often great, but it is chiefly in the caudal
region that the excess occurs; there being 36 caudal vcrtebrce in the crocodile, and 115 in the monster lizard. In the serpents, the vertebral column is more abundantly subdi vided than in any other animal; the number of vertebrae in the python being 422, of which about six-sevenths possess ribs articulated to their bodies by a ball-and-socket joint. By the motion which is thus allowed to the ribs, they become in some degree instruments of progression. In the reptiles generally (excepting the tortoises), one sur face of each centrum (or body) of the vertebra is concave and the other convex; while in the tortoises these surfaces are flat. The true skull is small, the bulk of the head being made up by the jawbones. As the sutures separating the individual bones .
never become obliterated, the reptilian SRull is well adapted to illustrate the true struc ture of the vertebral skeleton. In Fig. 1 we have the skull of the crocodile. 1 is the principal frontal, divided in the serpent into two parts; 2, 2, are the anterior and 4, 4 the poskrior frontals; 7 is the parietal bone,which is usually single in reptiles, 12, 12 are the mastoid bones (homologous to the mastoid TIrtir.C.QC 17 17 are the iwinrInforilbr_ ries; 18, 18 are the maxillaries; 20, 20 are the nasals; 23 is the temporal bone (corresponding to the squamous portiou of the human bone); 34, 35, 36, 37, are the dental, the articular, the angulae, and the opercular portions of the inferior maxilla, or lower jaw ; a is the tympanic bone, which supports the drum of the ear; b is the zygomatic or malar bone: and c, c the lachrymal& The lower jaw (except in the tortoises) presents the peculiarity of being composed of a number of sepa rate pieces; there being four or five in each half-jaw in serpents, while in crocodiles and lizards each half is divided into at least five, and generally six pieces, which are united by suture. The four important of these are shown in Fig. 1. The purpose of this arrangement is probably (as Dr. Buckland suggested in his Bridgewater Treatise) to diminish the risk of fracture, which would otherwise attend the snapping together of their elongated jaws.