The ribs have capitula and tubercula, and often uncinate processes (see p. 190) as in birds. A pectoral girdle and sternum, with clavicles and a T-shaped interclavicle are developed, and abdominal ribs are always found. The precoracoid is however absent. The limbs are pentedactylate.
Sphenodon[75] (Hatteria) now living in some of the islands of the New Zealand group, is certainly the most generalised of all living reptiles. Though lizard-like in form it differs from all living lizards in the possession of two temporal arcades, abdominal ribs and a fixed quadrate; and is often considered to be nearly allied in many respects to the type of reptile from which all the others took their origin.
Among the better known extinct forms are Proterosaurus of Permian and Hyperodapedon of Triassic age.
Order 6. Squamata.
This order includes the extinct Mosasaurians, and the lizards and snakes which form the vast majority of living reptiles. The trunk may be moderately elongated and provided with four short limbs as in lizards, or it may be limbless, extremely elongated, and passing imperceptibly into the tail. The surface is generally completely covered with overlapping horny epidermal scales, below which bony dermal scutes may be developed.
The vertebrae are procoelous, rarely amphicoelous. There are no inter centra, and the neural arches are firmly united to the centra. Additional articulating surfaces, the zygosphenes and zygantra, are often developed[76]. The sacrum is formed of two or rarely three vertebrae, or may be wanting as in Ophidia. In the skull an infratemporal arcade forming the lower boundary of the infratemporal fossa is absent, and the quadrate, except in the Chamaeleons, is movably articulated to the squamosal. The palatal vacuities are large and the nares are separate. There is often a distinct parasphenoid. The teeth are either acrodont (i.e. ankylosed to the summit of the jaw), or pleurodont, i.e. ankylosed to the inner side of the jaw. The thoracic ribs each have a single head which articulates with the centrum of the vertebra; while uncinate processes and abdominal ribs never occur.
A pectoral girdle and sternum may be present, or may be completely absent as in snakes. Except in snakes there are generally four pentedactylate limbs which may either form paddles or be adapted for walking.
Suborder (1). Lacertilia[77].
The body is elongated, and as a rule four short pentedactylate limbs are present, but sometimes limbs are vestigial or absent. The exoskeleton generally has the form of horny plates, spines, or scales; while sometimes as in the Chamaeleons and Amphisbaenians it is absent. In other forms such as Tiliqua and Scincus, the body has a complete armour of bony scutes, whose shape corresponds with that of the overlying horny scales.
The vertebrae are procoelous, rarely as in the Geckos amphicoelous; they are usually without zygosphenes and zygantra, but these structures occur in the Iguanidae. The sacral vertebrae of living forms are not ankylosed together, and the caudal vertebrae usually have well-developed chevron bones.