Reptiles, a very large class of Vertebrate animals, including Tortoises and Turtles, Lizards of many kinds, the divergent New Zealand 'lizard' Sphenodon, Snakes, and Crocodilians—five distinct orders with living representatives, but including also at least as many orders of wholly extinct types, such as Ichthyosaurs, Plesiosaurs, and Deinosaurus.
Reptiles occupy a central position in the Vertebrate series: beneath them are Amphibians and
Fishes, above them are Birds and Mammals. They begin the series of 'higher Vertebrates,' which at no period of life breathe by gills, which in embryonic life are provided with two birth-ropes or fetal membranes—a protective amnion and a respiratory allantois. Their relationships seem to be threefold, with the Amphibians, with Birds, and with Mammals. But there is no doubt that they are most closely linked to Birds—a fact first clearly recognised by Huxley, who emphasised the deep structural affinities of Birds and Reptiles by linking them together as Sauropsida, in contrast to Mammalia on the one hand and Ichthyopsida (Amphibians and Fishes) on the other.
Referring to the article BIRDS for a contrast between Mammals, Birds, and Reptiles, we shall simply notice that Reptiles are cold-blooded, the temperature of the body not greatly exceeding that of the surrounding medium; that the heart is three-chambered, except in Crocodilians, where four chambers first occur; that mostly venous blood goes from the heart to the anterior viscera, and mixed blood to the posterior region, only the head and anterior regions receiving purely arterial blood; that the body is covered with scales, with which subjacent bony plates or scutes are sometimes associated; that the skull articulates by a single condyle with the backbone, and the lower jaw works against the quadrate bone; that the great majority are oviparous, while in some the eggs are hatched within the mother. The earliest remains of Reptiles are found in Permian strata, and the golden age of Reptiles was in Mesozoic, especially in Jurassic and Cretaceous, times.
Classification of Living Reptiles.—As the orders of Reptiles with living representatives are separately discussed, it will be enough here to give a general classification. Order 1, Chelonia: Tortoises and Turtles. 2, Rhynchocephalia: one form—the New Zealand lizard Sphenodon (q.v.), whose extinct ancestors date from the Permian. 3, Lacertilia: Lizards (q.v.). 4, Ophidia: Snakes (q.v.). 5, Crocodilia: Crocodiles (q.v.), Alligators (q.v.).
EXTINCT REPTILES.—The classification of the extinct Reptilian types is still very uncertain; but many authorities agree in recognising the following orders:
Anomodontia.—Reptiles with lizard-like body, limbs adapted for walking, biconcave vertebrae, and teeth fixed in sockets. The order is restricted to the Permian and Trias, and exhibits affinities with the Labyrinthodont Amphibians and with Mammals. Among the representative genera are Pariasaurus, Galesaurus, Deutosaurus, Dicynodon, and Placodons.
Sauropsitygia.—Reptiles without exoskeleton, with long neck and short tail, limbs adapted for walking or for swimming, biconcave vertebrae, teeth fixed in sockets. All of them seem to have been carnivorous. The order is represented from the Trias to the Upper Chalk, and exhibits affinities with Amphibians. Among the representative genera are Plesiosaurus, Mesosaurus, and Nothosaurus.
Ichthopterygia.—Marine Reptiles, with whale-like body, without exoskeleton, with limbs modified as paddles, with biconcave vertebrae, with teeth implanted in a continuous groove. Many were carnivorous and fed on fishes. Seeley has shown that some were viviparous, the fossilised young being found in the fossilised mothers. The order is represented from the Upper Trias to the Upper Chalk, and exhibits affinities with Labyrinthodont Amphibians and with the New Zealand 'lizard' Sphenodon. Among the genera are Ichthyosaurus and Ophthalmosaurus. Some attained a length of 30 to 40 feet.
Rhynchocephalia.—As Sphenodon is the only surviving representative of the Rhyncocephalia, the order may be almost regarded as extinct. It is represented by Palæohatteria from the Permian, besides Champsosaurus, Hyperodapeton, Rhynchosaurus from later strata. Within or near this order may also be included a remarkable form Proterosaurus from the Permian, a type for which Seeley has established a distinct order, Proterosauria. The special interest of these forms is, according to Baur, that they 'are certainly the most generalised group of all reptiles, and come nearest, in many respects, to that order of reptiles from which all others took their origin.'
Dinosauria.—The largest land Reptiles of crocodylian or more bird-like form, represented from the Trias to the Upper Chalk, exhibiting affinities with crocodiles and with birds. Representative genera are Iguanodon (sometimes measuring about 30 feet), Camptosaurus, Seelidosaurus, Stegosaurus, Ceratops (with long horns on the skull), Megalosaurus, Ceratosaurus (also horned), Brontosaurus (upwards of 50 feet in length), Atlantosaurus (with a femur 6 feet long).
Ornithosauria.—Flying reptiles, often called Pterodactyles, with a fold of skin extended on the greatly elongated outermost finger. The order is represented from the Lias to the Upper Chalk by such genera as Pteranodon, Pterodactylus, Dimorphodon, and Rhamphorhynchus. Some had an expanse of wing of about 25 feet, but many were small. Their affinities are uncertain.
See Huxley, Anatomy of Vertebrated Animals (1879); Nicholson and Lydekker, Manual of Palæontology, vol. ii. by Lydekker (Edin. 1890); Hoffmann in Bronn's Klassen und Ordnungen des Thierreichs (Leip., in progress); Duméril and Bibron, Erpétologie Générale (9 vols. Paris, 1834-54); British Museum Catalogues by Boulenger &c.; Holbrook, North American Herpetology (Phila. 5 vols. 1836-42).