Reptiles (Lat. sing. reptilis, from repere, to creep), a class of vertebrated animals intermediate between fishes and birds. Linnaeus united the oviparous quadrupeds and the serpents of Aristotle under the erroneous name of amphibia; until within a recent period batrachi-ans, as well as serpents, lizards, and tortoises, were included among reptiles, but now the first are regarded as a distinct class. As thus limited, reptiles do not undergo metamorphosis, are always air breathers, though coldblooded, and have neither mammae, hair, nor feathers. By the first two peculiarities they are distinguished from fishes and batrachians, and by the third from mammals and birds. Although they breathe air by lungs like birds and mammals, the pulmonary circulation is incomplete, only a part of the blood being sent to them, and, from the communication of the ventricles of the heart or the great vessels, a mixed arterial and venous blood, principally the latter, is sent to the organs. Reptiles have been divided into chelonians or tortoises, saurians or lizards, and ophidians or serpents, whose characters are given under their respective orders, families, and popular names.

The various systerms of classification will be found under Her-petology; the batrachians have been treated under Amphibia, and the anatomical peculiarities of the order under Comparative Anatomy. The number of species of reptiles is about 2,000, or less than that of mammals or birds; most of them are terrestrial, but some (as the dragons) can sustain themselves in the air like the flying squirrels, and the extinct pterodactyl probably winged its way like the bats; some live habitually in the water, swimming by means of flattened fins (as the turtles), or by a laterally compressed tail (as in croco-dilians); the amphisbaena and other ophisau-rians dwell in subterranean burrows. They present every degree of speed, from the agility of the lizard to the slowness of the tortoise; some are fitted for running over dry sand, others for climbing trees, others for ascending smooth surfaces; the limbs are not generally adapted for rapid or graceful motions, being short, almost at right angles with the spine, and hardly raising the body during locomotion enough to prevent the ventral surface from dragging on the ground; the anterior limbs are the shortest, and the knees and elbows are constantly flexed and far apart longitudinally; the feet are not adapted for prehension (the chameleon excepted), so that they display little skill in preparing retreats for themselves or places for their eggs.

They are naturally coldblooded for reasons given below, and are found in greatest abundance and of largest size in warm climates; under the influence of cold they pass into a lethargic state, and according to Humboldt a similar condition befalls the South American crocodilians during the hottest season of the equatorial regions. The tortoise and the crocodile are sufficiently protected against ordinary enemies; the lizard darts into its hole, perhaps at the expense of a part of its tail, which is soon reproduced; the great boas prevail over every foe but man; many serpents are armed with poisonous fangs, rarely used however except on the defensive; some are covered with bristling spines, like the horned lizards, and are thus saved from predaceous animals. They are of great use to man in destroying noxious insects and other animals; some, like the chelonians, furnish a wholesome and abundant food, and others supply various articles useful in the arts. They are preyed upon by carnivorous birds, as eagles, storks, cranes, and the ibis, and by such mammals as the ichneumon, hog, and the smaller carnivora; they are themselves essentially carnivorous, and feed on living prey which they swallow whole, but the marine turtles are principally herbivorous. - The osteology of reptiles has been given sufficiently in the various articles above referred to.

Except in chelonians, the form is generally elongated, more or less cylindrical, with a very long tail; the feet are absent in serpents and in some saurians, and the others; the skeleton is always osseous, the cranium small, and the facial bones and jaws greatly developed, the latter usually armed with sharp, hooked teeth; the toes are freely movable, and usually with strong claws, webbed in the crocodiles and turtles. The body is covered with scales, generally appendages of the true skin; the overlying epidermis is cast off periodically; the scales are converted into bony plates in the chelonians and crocodiles, and in lizards and serpents are often brilliant with metallic reflections; in the chameleon, anolis, etc, the surface modifications of the skin present very rapid changes of color, sometimes expressing the anger or fear of the animal, and in some cases enabling them to avoid detection by their enemies. The muscles of reptiles are red, though paler than in mammals and birds; they preserve their irritability for a long time after the death of the animal, in chelonians even after many days; tortoises have been known to live for 18 days after the removal of the brain, groping blindly about.

The brain is small, with cerebrum, cerebellum, and medulla oblongata; they have also a spinal system of nerves, and a sympathetic or ganglionic chain; in most the spinal marrow is relatively much more developed than the brain, the latter being smooth, without convolutions, the cerebral lobes being the largest; the cerebral hemispheres contain lateral ventricles, and are larger than the optic lobes, which in fishes constitute the greater part of the brain; there is no pons Varolii, and the cerebellum is more developed than in fishes. Life seems in a remarkable degree independent of the brain, the class rather vegetating than living, and being comparatively insensible to pain; they grow slowly and live long, and are exceedingly tenacious of life; the intelligence is hardly greater than in fishes. The sense of touch is dull, both active and passive, and whether exercised by the skin, toes, lips, tongue, or tail; taste must also be dull, as the food is swallowed without mastication, and the sense of smell must be still less. The organ of hearing is less developed than in birds and mammals; there is no external ear; the tympanum, where it exists, is bare and almost external, and the internal ear is less developed than in fishes.

The eyes are usually small, occasionally absent, flat, with incomplete bony orbits, with lids (except in serpents), and with lachrymal glands. The nasal cavities are large, and always communicate with the mouth, and in the crocodiles are very far back. The lungs are sometimes large, extending even through the whole length of the ventral cavity, which has no diaphragm; in the long-bodied snakes only one lung is active, the other being very rudimentary or absent; these organs are comparatively free, the trachea not divided into bronchi, and the air cells few, large, and freely communicating with each other; in lizards and serpents the ribs serve for respiration, and in tortoises the scapular arch performs the office of ribs, according to Van der Hoeven, respiration not being effected by deglutition. Only a small portion of the blood is sent to the lungs, and this is feebly oxygenated, as the respiration is performed slowly and the lung is of loose texture and small capacity; hence a low degree of animal heat, languid movements, and a slow performance of the nutritive functions. They have no true epiglottis and no proper voice, though some emit a hissing sound (as the ophidians) formed in the mouth.

The heart has four cavities, but the ventricles communicate, except in the crocodilians, where an admixture of the arterial and venous bloods takes place in the great vessels; there is, therefore, a partial circulation, independent of respiration, enabling them to remain long under water and in irrespirable gases. The lymphatic system is greatly developed, having regular pulsating organs or lymphatic hearts for the propulsion of their fluid. Reptiles eat and drink comparatively little, and are able to go a long time without food; not having movable and fleshy lips, they cannot perform the act of suction, as was once popularly believed of serpents; the mouth is generally large, and the lower jaw articulated by a distinct bone, the homologue of the os quadratum of birds. The tongue is generally free, and the oesophagus very wide and distensible to accommodate large prey; the intestine is short and straight in proportion to the carnivorous disposition, being longest in the herbivorous chelonians and shortest in the snakes; there is a certain division into small and large intestine, though the latter in most is properly the rectum; the alimentary canal opens below into a cloaca, or cavity common to the digestive, urinary, and reproductive organs, as in birds; all the nutritive elements are extracted from the food, the indigestible matters being ejected in a mass at long intervals; the vent is transverse in snakes and lizards, but longitudinal in chelonians and crocodiles, corresponding to remarkable differences in the male external reproductive organs, these in the former being double and placed in a cavity behind the anus, and in the latter single and within the cloaca.

Salivary glands, which are absent in fishes and batrachians, are present in reptiles; the liver is always present and large, receiving much venous blood, especially that from the posterior part of the body; the gall bladder is commonly found, though small; the spleen is generally very small, removed from the liver and stomach, rounded, and deep red; the pancreas is constant, often large at the beginning of the intestine, and of various forms; the kidneys are situated along the spine, showing no distinction of cortical and medullary portions; the ureters open into the cloaca, and the urine is a whitish mass, more or less hard, containing salts of lime and ammonia; the supra-renal capsules are usually present, small, and often remote from the kidneys; there are one posterior and two anterior venae cavae. The power of reproducing lost parts is less than in batrachians, and is noticed especially in the tails of certain lizards and serpents. In this class there is no durable union of the sexes as in birds and mammals, and nothing which exerts any influence on the social condition of the individuals; after the instinctive act of reproduction they separate and become perfect strangers.

Most are oviparous, leaving their eggs to be hatched by the heat of the sun, and the young when born are able to provide for themselves and are generally indifferent to the mother; the female rarely makes a nest, but deposits her eggs in a safe, warm, and dry place; crocodiles and some lizards watch in the neighborhood of the place where their eggs are concealed, and the python has been seen in menageries coiling herself around her eggs in a conical form, closing the top with her head. Some of the serpents are viviparous, the young being so far developed before the exclusion of the eggs as to be born alive; in the viviparous snakes the young are said to take refuge within the mouth of the mother. The eggs have generally a more or less calcareous shell, globular or rounded equally at each end; in serpents they are often joined together in chaplets; their number varies from 20 to 100. The embryo is completely enveloped by the amnios, and after it has attained a considerable degree of development a second membranous covering appears, for the first time in vertebrates, the allantois, richly supplied with vessels and enclosing embryo and amnios. - As reptiles are generally despised and hated by man, and comparatively little under his influence, their original geographical distribution has been but slightly changed by him.

Most of the serpents, especially the venomous kinds, belong to warm regions. - The secondary geological epoch, comprising the trias, Jurassic, and chalk, has been called the age of reptiles; during this period air-breathing animals first appeared in considerable numbers, and reptilian forms predominated. Reptiles are connected with birds, especially those of the former called symphy-poda by Cope; dinosaurians, progressing by leaps, with very small anterior limbs, have made many of the bird-like tracks described by Hitchcock in the sandstone of the Connecticut valley. The gigantic and uncouth forms of the secondary age had disappeared in the tertiary, and the reptiles of the latter were more like the present ones, except in geographical distribution, and were in about the same proportion to the rest of creation as now. The study of fossil reptiles shows the limited duration of species; before the diluvial epoch there is not a single reptile that can be referred to living species and hardly to an existing genus; the reptiles of each age, triassic, Jurassic, and cretaceous, have a special facies, unlike any which preceded or followed them; the difference between the fossil and living forms is always greater as we go back in time.

This study also proves that the temperature of the earth has varied, as the great reptiles above named lived in parts of Europe nearer the frigid than the torrid zone. All the fossil forms, however odd, were constructed on the same reptilian vertebrate type as at present, in some instances with ornithic (pterodactyl) or mammalian affinities (ichthyosaurus). In the most ancient fauna of reptiles, chelonians and sau-rians, the highest in the class, are represented, and some forms then existing were in certain respects more perfect, or at any rate more complex, than some present members of the class; each fauna had its type of perfection, without regard to the superiority-or inferiority of that which preceded or followed it; we find no transition species leading to or from ichthyosaurus, plesiosaurus, pterodactyl, and the like, unless we ascend to cetacean mammals in the first and to bats in the last. Reptiles (including batrachians even) are very rare, and to some questionable, in the Devonian; there are a few amphibians in the carboniferous; the class abounds in the divisions of the trias, and is most numerous in the Jurassic, becoming less abundant in the oolite and chalk. For details on fossil reptiles see the various articles on the genera above mentioned.

Professors Cope and Marsh have described several new forms of reptiles from the western territories, in the "American Naturalist," "Proceedings of the Academy of Natural Sciences," Philadelphia, and the "American Journal of Science".