The vertebrae are usually procoelous, but may be amphicoelous; there are two sacrals or none. Ribs are single-headed throughout. The shoulder girdle, if present and fully developed, consists of scapulae and precoracoids, often enlarged and notched or fene strated, clavicles, an interclavicle and a sternum. The pelvis has a forwardly and downwardly directed ilium, the pubes and ischia are divergent rods. The limbs are pentadactyl primitively, but may be reduced or absent.
Sub-order Lacertilia (Lizards). Squamata in which a temporal arcade is usually present, and in which the two rami of the lower jaw are connected suturally at the symphysis. An epipterygoid is present in the normal position and the anterior part of the brain case is very little ossified. The pterygoid articulates with the basipterygoid process of the basisphenoid.
Upper Jurassic to Recent.
Division Ascalabota.
Section Gekkota.
Families: Ardeosauridae, Gekkonidae, Uroplatidae.
Section Iguania. Families : Iguanidae, and Agamidae.
Section Rhiptoglossa. Family Chamaeleonidae.
Division Antarchoglossa. Section Scincomorpha. Families: Xantusiidae, Scincidae, Anclytropiidae, Flyliniidae, Dibamidae, Gerrhosauridae, Lacertidae, Tejidae, Amphisbaenidae. Section Anguimorpha. Families : Euposauridae, V araniidae, Dolichosauri dae, A igialosauridae, Mosasauridae, Pygopodidae, Glyptosauridae, Helodermatidae, Anguidae, Xenosauridae, Anniellidae, Zonuridae.
Sub-order Ophidia (Snakes). Squamata in which the temporal arcade has completely vanished, and the quadrate is very freely movable. The pterygoids have lost all connection with the basi sphenoid, and the palate has become mobile, connected to the cranium only by ligaments and by its connection with the maxillae and quadrate. Much of the palate and the maxillae may vanish in burrowing forms. The brain case is completely ossified, the epipterygoid being absorbed into it. The two halves of the lower jaw are loosely connected by an extensible ligament.
The vertebral column is extraordinarily long, in one case con taining 565 vertebrae. Each vertebra has a procoelous centrum and a heavy neural arch, on which additional articulating faces, the zygosphenes, and zygantra, are developed. The single-headed ribs are long and are very freely movable antero-posteriorly ; by such movements they cause the transversely widened ventral scales to catch the ground, and force the animal along. There is never
any trace of a fore limb or its girdle. All three elements of the pelvic girdle may be present in one family, the Glauconidae, but in most this limb is entirely absent. Upper Cretaceous to Recent.
Families : Typhlopidae, Glauconiidae, Ilysiidae, Uropeltidae, Boidae (boa constrictors), Xenopeltidae, Colubridae. As the last family contains nine-tenths of all known snakes it is subdivided into the series Aglypha (harmless snakes), Opisthoglypha (poi sonous but little dangerous to man) and Proteroglypha (typical poisonous snakes).
Sub-order Pleurosauria. A small group of extinct reptiles in cluding only one or two genera, which may be related to the Squamata ; if so, these are not, as usually held, derived from the Archosauria. Aquatic reptiles with a very long body and lizard like limbs partially adapted for swimming. Limb girdles of Lacertilian type. Skull elongated and depressed, quadrate short and immovable. There is a single temporal fossa, bounded below by a broad arcade composed of the squamosal, postorbital and jugal. There is no supratemporal, and the outer surface of the quadrate is covered by a quadrato-jugal. Upper Jurassic. One family.
Order Sauropterygia (Plesiosauria). Reptiles which show a progressive adaptation to a marine life.
Skull with a single temporal vacuity surrounded by the parietal, squamosal, postorbital and post-frontal, and there fore apparently homologous with the upper temporal vacuity of Rhynchocephalia, and not with the single fossa of Thero morpha and Squamata. The single temporal arcade is formed almost entirely by the squamosal and postorbital, the jugal being a small bone wedged in between the postorbital and the hinder end of the maxilla. A quadrato-jugal is absent. The fenestra ovalis lies high in the side wall of the brain case. The palate is primitive, the posterior nares being anterior, and the pterygoids reaching the prevomers.
Except in Placodonts, the neck is long, often exceedingly so (76 vertebrae in Elasmosaurus), the back is long and the tail short; there are usually three sacral vertebrae, but may be more. The cervical ribs, though double-headed in early forms, articulate only with the centra, the single-headed dorsal ribs being supported entirely by the long transverse process of the neural arch.