REPTILES, or REPTILIA, a class of vertebrate animals. Although no two groups of Vertebrata are more dissimilar in general appearance and habits, and to the popular mind might seem more unrelated than the birds and reptiles, they are, by common consent of modern systematists, combined in a superclass, the Sauropsida, of Huxley, or the Monocondylia, of Haeckel and Cope. The Sauropsida are dis tinguished by the possession of lungs, gills be ing permanently absent. The embryo possesses an amnion and allantois, the former being a sac investing the body, the latter a structure de veloped from the lower surface of the embryo and serving for respiration. The lower jaw in Sauropsida is compound in that each half or ramus consists of a number of distinct pieces; while in mammals each half consists of but a single piece. The lower jaw in reptiles and birds further articulates with the skull, not di rectly, but through the intervention of a sepa rate and distinct bone, the os quadratum or quadrate bone. The skull is joined to the spine by one condyle only; two such condyles exist in amphibians and mammals. As compared with the fishes and batrachians the basal axis of the brain-case is bony instead of cartilaginous and the parasphenoid bone is absent. The bodies of the vertebra are formed chiefly of centra (that is, are gastrocentrous) instead of intercentra (notocentrous) as' in the Amphibia. Very rarely and only in extinct forms is the notochord persistent. The anklejoint is placed in Sauropsida, not as in mammals, between the tibia and the astragalus, but between the proximal and distal parts of the tarsus which in Sauropsida becomes thus divided. The cora coid bone is almost always well developed. The intestine ends in a cloaca. No complete dia phragm is developed to separate the thorax from the abdominal cavity. The corpus cal losum uniting the halves or hemispheres of the cerebrum is rudimentary, no mammary glands exist and the Sauropsida are oviparous or ovo-viviparous. Such are the characters common to birds and reptiles.
Comparison with Reptiles differ from birds in the following characters, which may, therefore, be taken as including the defini tion of the class Reptilia. The covering con sists of horny scales or of bony plates (scutes), but never of feathers. The blood is cold and two aortic arches (right and left) exist in liv ing Reptilia. The heart is three chambered in all save the crocodiles, which possess a four chambered heart. But in all reptiles, without exception, the venous and arterial currents of blood are connected and an impure or mixed blood is thus circulated throughout the body. The lungs do not present the open character of those of birds, but, like those of marmnalia, are in modern reptiles almost always closed sacs. The tarsal and metatarsal bones of the hind limbs, which in birds are united to form a single bone are distinct and separate in the great majority of reptiles. When a sacrum exists it bears sacral ribs, which articulate with the ilia or haunch-bones.
The body in reptiles is generally elongated, the tortoises and their allies present ing the most notable exceptions to this rule. The limbs may be entirely wanting, as in most snakes and in many lizards; or only a pair of limbs may be developed, as in some lizards; while in mort other reptiles all four members are present. The bones of reptiles are more compact than those of lower vertebrates. In the reptilian skeleton the five different regions into which the spine is ordinarily divided are to be recognized, except in the serpents and a few others. There are seldom more than two sacral vertebra, and free cervical ribs are usu ally present. The epiphysial and other sutures of the vertebra are retained through life. The bones of the shoulder-girdle of each side in clude a simple or divided scapula or shoulder blade and an often complex coracoid bone, in cluding precoracoid and epicoracoid, the latter bone of each side articulating with the ster num or breast-bone when the latter is present.