Cold-blooded Sauropsida with epidermal scales, sometimes combined with underlying dermal bones and then forming scutes; and not developed as are hairs and feathers in saccular involutions of the integument. The characters of the skeleton vary much but the vertebral centra never possess the typical Avian shape. The sacral vertebrae are csually two, provided with large ribs expanded distally. The inter clavicle, when present, never fuses with the sternum which is usually rhomboidal, cartilaginous and more or less ossified but not, as in birds, replaced by membrane bone (? Pterodactyles). The ilia vary much in the degree to which they extend forwards, and in existing Reptilia usually extend backwards more than in the opposite direction: the ischia form a ventral symphysis as do the pubes except in certain of the extinct Deinosauria. The pelvic bones and metatarsalia remain separate except in the extinct Ceratosaurus in which they anchylose. There are always two aortae (right and left), and the ventricle is either single, or double as in Crocodilia where however the venous and arterial blood currents communicate at the base of the aortae: there are only two semilunar valves at the bases of the aortae and pulmonary artery.
The outer layer of epidermic cells is cornified and its cells often contain air. It is thrown off and renewed periodically in Lacertilia and Ophidia. The integument appears to have been smooth in the extinct Plesiosauria and Ichthyosauria, and in the Amphisbaenoidea among Lizards it is divided into rectangular areae disposed in transverse rows. Scales are formed as duplicatures of the integument. The corium (derm) is composed of horizontal and vertical bundles of fibres as in the lower Vertebrata. Pigment is not usually found in the epidermis but in special pigment cells of the corium and in many Lacertilia (e.g. Anolis, Chamaeleon) there is a change of colour apparently connected with psychical changes. Dermal bony plates occur in many Lacertilia (e.g. many Scincoided), often of microscopic size as in the Geckoes (Ascalabota). In the Crocodilia these dermal plates are of large size and form scutes placed on the neck, back and sometimes on the ventral surface, or, as in the extinct Teleosaurus, covering the body completely. In the Triassic Aetosaurus they overlap one another. Among the Deinosauria the Stegosauria have large bony plates with spines.
The dermal skeleton of the thorax and abdomen is largely developed in the Chelonia and forms a firm carapace and plastron within which the head and neck, the tail and limbs can be withdrawn in many instances for shelter. The carapace consists of a median series of eight ossifications which, with the exception of the first and two last, unite each with the dorsal spines of thoracic vertebra; of a lateral series of costal plates united with the ribs, and a marginal series sometimes absent (e.g. in Chelone niidas) into which the ends of the ribs are received. The plastron is imbedded in the corium and consists of a median anterior entoplastron, and four pairs of plates (hyo-, hypo-, xipho-plastra) placed one behind the other. These plates cover the ventral surface more or less completely. In many Chelonia the carapace and plastron unite firmly at their lateral edges, e. g. land Tortoises. The abdominal walls are covered by a system of dermal bones in the Plesiosauria and Ichthyosauria, arranged in transverse rows; each row consists of a median bone pointed at both ends and at least three overlapping bones on either side. Most Reptilia possess claws. The integument is poor in glands.
Scent-glands are present beneath the mandibles, opening on the neck, or at the sides of the plastron in Chelonia. The Ophidia have a pair of cloacal scent-glands, and the Crocodile similarly placed gland follicles. Among Lacertilia, Hatteria has a pair of cloacal glands, and in the majority there is a row of ventrally placed femoral glands, the secretion of which appears to have a copulatory function.
The conformation of the cranial bones varies much in the different groups. The cartilage cranium is much reduced. The occipital region, basisphenoid and alisphenoid are ossified; but the presphenoid and orbito-sphenoid are cartilaginous or imperfectly ossified and the ethmoidal region is always unossified. There is an interorbital (ethmo-presphenoid) septum present except in Ophidia. The pro-otic is usually free: the epi- and opisth-otic fused with the supra- and ex-occipitals respectively: but in Chelonia and Ichthyosauria the last-named remains free. The palato-pterygoid region is well ossified and large in Chelonia and Crocodilia. The series of bones developed in membrane is very complete. There are frequently prae- and post-frontáls present in front of and behind the orbits respectively. The nasal bones are absent in post-cretaceous Chelonia. In Lacertilia the quadrato-jugal is represented by ligament: it is ossified however in Hatteria. The parasphenoid is very large in Ophidia, and the trabeculae cranii remain as two rods on either side of it in front of the basisphenoid and behind the ethmoid.
The vomers are double in Lacertilia and Ophidia. The quadrate is immoveably united to the skull in Chelonia, Crocodilia, in Hatteria and Chamaeleons among Lacertilia, and in the extinct Icthyosanria and Plesiosauria. The bones of the lower jaw usually retain their sutures: but in Ophidia the articular, coronoid, angular and surangular are anchylosed. The rami are anchylosed at the symphysis in Chelonia; connected by a distensible ligament in Ophidia, or united by suture, or cartilage. Iguanodon, Hypsilophodon and Diclonius among Deinosauria, possess a prae-symphysial or mento-meckelian bone in front of the symphysis. The stapes (columella auris) is a simple rod-like bone. The hyoid arch is well-developed and the Chelonia have a large remnant of the first branchial arch.
Amphicoelous vertebrae are found in the existing Hatteria and Geckoes among Lacertilia: in the prae-cretaceous Crocodilia, e. g. Teleo-saurus: in Ichthyosauria, Plesiosauria and many Deinosauria. In these the notochord is enlarged intervertebrally. In other Reptilia it is enlarged intra-vertebrally and constricted inter-vertebrally and eventually lost. The centra are usually procoelous and are connected by synovial joints, or in the Crocodilia by intervertebral discs. Opisthocoelian or biconvex centra are found in exceptional instances, concavo-convex in many Deinosauria; and the mobility of the neck in Chelonia is insured by a great variety in the shapes of the centra of the cervical vertebrae. The neural arches are united to the centra by anchylosis in Ophidia, Lacertilia and most Chelonia-. by suture in a few Chelonia, Crocodilia, the extinct Plesiosauria; and by interposed cartilage in the Ichthyosauria. In the last named order, the vertebrae articulate only by their centra; in the remaining orders by the usual articulating processes, and in Ophidia, as well as in the Iguanidae among Lacertilia by a zygosphene and zygantrum in addition (see p. 73). In Ophidia, in Amphisbaenoidea among Lacertilia and the Ichthyosauria the vertebral column is divisible into a praecaudal and caudal series (see p. 72): in other orders into a cervical, dorsal, lumbar, sacral, and caudal regions, but the lumbar region is absent in Chelonia. The number of vertebrae in each region varies much in the different orders.