In addition a clavicle and interclavicle may be present. The fore-limbs consist each of a humerus, radius and ulna, the carpal bones and normally five digits. The pelvis contains its three typical elements, ilium, ischium and pubis, the first being, often of larger size and the last usually forming a symphysis. In ser pents the shoulder-girdle is totally absent and vestiges of the pelvic-girdle appear only in a few instances; and in the turtles both are en closed within the shell, except that the clavicle and interclavicle form part of the plastron. The limbs are usually'adapted for running, but some times for leaping and in the extinct pterodactyls for flight. In the extinct marine pelagic rep tiles, and in the modern sea-turtles, they are paddle-like. The ribs in reptiles are always present, but may differ greatly in form and dispostion, those of the turtles, for example, contributing toward the formation of the upper shell or carapace.
The skull of Rettilia possesses but a single occipital condyle, which is sometimes tri partite. The quadrate bone is generally firmly fixed to the skull, joining the squamosal bone, but is freely movable in serpents and only less so in many lizards. Each half or ramus of the lower jaw is composed of dentary, angular, sur angular, coronoid, splenial and articular bone. Other regions of the skull are modified in the greatest varieties of ways in the several orders, the most important from a systematic stand point being in the bones which form the com plex roof of the temporal fossa. Primitively this roof is complete as in the stegocephalian batrachians, but by the formation of one or two openings (supra- and infra-temporal vacuities) it is divided into one (Synapsida) or two (Diapsida) longitudinal bars behind the orbits. Through variations in the size and position of these openings, to which may be added emar ginations, this region is modified, and may even practically disappear altogether, as in the snakes.
Digestive The teeth are gen erally well developed, but in the Chelonia are wanting, the jaws, like those of birds, being ensheathed in horn. The reptilian teeth, like those of lower V ertebrata generally, are adapted less for mastication than for merely retaining prey while it is being swallowed. Save in the crocodiles and in some extinct forms the teeth are not implanted in sockets or alveoli, but are attached in various ways and by bony union to the jaw-bones. Teeth may be borne by bones other than the jaws (for example, palate bones) ; and, as seen in the poison-fangs of serpents, may be modified for special purposes. The teeth vary greatly in number and are not sistent, new teeth being produced in regular order (as in crocodiles) from a growing pulp at the base of the socket, the new tooth dis placing the old.
The tongue may be elongated, distensible and bifid, as in many lizards and serpents; short, thick and non-protrusible, as in other lizards; or completely attached and fixed throughout its entire extent, as in Crocodilia.
The oesophagus or gullet is usually greatly distensible (as in serpents), and may be cov ered (as in some Chelonia) with retroverted spines. The stomach is mostly pyriform or pear-shaped, and (as in snakes) may, like the gullet, be capable of great distension. In Opkidio it exhibits an anterior dilated part, with thin walls for receiving nutriment, and a pos terior hinder portion with non-distensible walls provided with glands and adapted for digesting the food. In the crocodile the stomach re sembles the gizzard of a grain-eating bird in its high muscularity.
The heart in reptiles consists of two auricles (right and left) and a ventricle, except in the Crocodilia, in which two auricles and two ventricles are developed. The aorta or main arterial trunks exist, and are respec tively named right and left aorta. These chief vessels bend around the gullet and unite to form a single and common main-trunk for the sup ply of blood to the system generally. But the chief peculiarity in the circulation of all reptiles consists in the peculiar mixture of arterial with venous blood, which takes place in the aorta in such a manner that the head alone is supplied with arterial blood. The reptiles exhibit slug gish habits, slow respiration and a series of vital actions marked by no active conditions de manding a more perfect circulation or highly oxygenated blood. The blood is cold (poilcilo thermic) in reptiles, that is, but little higher in temperature than the surrounding medium, with changes in which it varies. The red blood corpuscles are oval and nucleated.
Respiration in reptiles is carried on solely by means of lungs, the presence of branchim or gills in early life, and sometimes in the adult life also of batrachians, constituting a marked difference between these latter and reptilian forms. In the Crocodilia, Chelonia and most lizards, the lungs are equally developed, but in serpents and some lizards only one lung is fully developed, the right lung being usually abortive. The lungs may, as in snakes and other Reptilia, in which the body is elongated, be of proportionally large size, and may extend nearly throughout the whole length of the body cavity which is not separated by a complete dia phragm into a distinct thorax and abdomen. In the crocodiles, lizards and serpents the respira tory action is carried on through movements of the walls of the trunks. But in turtles, in which no movements can take place, air is drawn into the lungs by a process analogous to swallowing. The larynx or organ of voice is simple in structure, no vocal cords being in general de veloped, although these organs appear in croco diles and some others.