at the same time the poison is expelled from the gland, partly by the contraction of the muscular walls of the gland, and partly by the muscles of the jaws, especially the temporals. This poison-gland is supposed to be one of the buccal salivary glands, modified in structure, so as to perform a different function; the poison secreted by it is a greasy, gelatinous, tasteless fluid, retaining its poisonous qualities for a long time if not exposed to heat, and drying in the air into scales; it is said to be harmless if swallowed. There are two or three other poison-fangs kept in reserve above and behind the one in use, ready to take its place if broken or destroyed. In most venomous reptiles there are no other teeth in the superior maxillary bones except the poison-fangs and their successors; in some, however, there are a few behind the poison-fangs, while all possess palatine teeth like the harmless snakes; lastly, in a few the terminal maxillary teeth are deeply grooved, but are not connected with the duct of any poison-gland. The ribs are very numerous and extremely movable, each terminated by a cartilage which is attached to the abdominal "scuta" by muscular connexions, and thus an ophidian reptile is said to walk on the extremities of its ribs. The heart has two auricles and one ventricle, the latter being imperfectly divided into two chambers by an incomplete septum. The lungs and other duplicate organs are generally symmetrical, one of each pair being rudimentary or absent. The urinary bladder is wanting; the aperture of the cloaca is transverse. The tongue is bifid and protrusible at will, and, when protruded, is in constant vibration; it is composed of two muscular cylinders united at their base, and is an organ of touch rather than of taste. The eye-ball is covered by a layer of transparent epidermis, attached circumferentially to a circle of scales (these scales are in lieu of eyelids), which is termed the "antocular membrane;" the posterior surface of this membrane is lined by a reflexion of the con

junctival covering from the globe of the eye, forming a cavity called the "antocular cavity," into which is poured the lachrymal secretion by which the eye is moistened. The antocular membrane is periodically shed with the rest of the epidermis, thus rendering the animal blind for a few days. In most snakes the pupil of the eye is round, but in the venomous serpents and boas it is a vertical slit. Ophidia are sub-divided as follows:

IV. CHELONIE. This order embraces a curious and interesting group of reptiles, comprising the turtles and tortoises. They are immediately recognized by the bony box which forms a protecting case for the body, within which the limbs and tail can almost always be withdrawn at

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will, and generally the head as well; this bony box is composed of two portions, an upper portion called the dorsal shield, or carapace," and a lower called the ventral, or "plastron." The carapace is formed by the spinous processes of the dorsal vertebræ, which are flattened out laterally, and constitute a series of broad plates, and by the ribs, which are also much expanded, forming what are termed the "costal plates." There are eight pair of ribs, generally, united along their lateral margins by sutures; in some instances, however, there are apertures at their extremities, which are covered by a thickened skin or horny plates. Along the margin of the carapace is placed a series of bones called "marginal plates," which are, by some, considered to be the ossified cartilages of the ribs, corresponding to the sternal ribs of birds, while others regard them as dermal bones belonging to the exoskeleton. The plastron, or ventral shield, consists of nine bony plates, of which eight are arranged in pairs, but the ninth is single, and occupies the mesial line, being always placed between the four pieces composing the two anterior pairs. By some the plastron is considered to be an enormously developed sternum, while others believe it to be composed of integumentary ossifications belonging to the exoskeleton. Both the carapace and plastron are covered, either with a leather-like skin, or, more usually, with a series of horny plates, which constitute in some species the "tortoise-shell" of commerce. All the bones of the head are immovably united together. In the adult, the lower jaw appears to consist of a single piece, as the different portions of which it had been composed in early life are firmly consolidated. The jaws are encased in horn so as to form a kind of beak; and teeth are entirely absent. The dorsal vertebræ, except the first, are immovably joined together, and there are no transverse processes; the cervical and caudal vertebræ present their wonted flexibility. The scapular and pelvic arches are, strange to say, situated inside the ribs, being

completely concealed within the bony box; the scapular arch consists of a clavicle, coracoid, and scapula, while the pelvic arch is formed by the ilium, ischium, and pubes. The heart consists of two auricles and a single ventricle, with an imperfect septum. The lungs are large, and respiration is effected by swallowing air, as in frogs. As most of the cheloniæ are phytophagous (pvтóv, a plant, and payev, to eat), the alimentary canal is consequently long and complex; and, to economise space, is generally arranged in a direction transverse to the longitudinal axis of the body. The cheloniæ are long-lived, can remain without food for a lengthened period, and will, it is said, live for months after the complete removal of the brain, while portions of their bodies retain their irritability for a long time. They are divided, in accordance with the structure of their limbs, into

Natatory Cheloniæ-e.g., all Turtles.

Amphibious Cheloniæ

e.g., Mud Turtles or Soft Tortoises (Tryonicida), Terrapins (Emydidae).

Terrestrial Chelonia-Land Tortoises.

CHAPTER VI.

AMPHIBIA.

AMPHIBIA (Batrachia, Cuvier), also termed diplopnæa (dɩπλî. double; and voń, breathing), are known by the following characters: They all possess branchiæ, which are either deciduous or permanent, hence, they have been divided into the caduci-branchiata and perenni-branchiata; true lungs are always present in the adult. There are always two occipital condyles for articulating with the vertebral column. The vertebral centra vary much, being amphicælous in the ophiomorpha (pis, a snake; and μopph, form), opisthocælous in the salamanders (σaλauavdpa, a lizard), and procælous in most of the others. When limbs are present, they are never converted into fins, but possess the same skeletal elements as those of the higher vertebrata. Their bones, both in chemical composition and in structure, are very similar to those of fishes; they have no medullary canal, no laminated arrangement, but the plasmatic canals exhibit slight dilatations, The heart, in the adult, consists of two auricles and a single ventricle. The nasal sacs communicate with the pharynx. The skin is smooth, glutinous, and almost always destitute of scales. There are never fin rays present, though median fins sometimes exist. There is a common cloaca. The embryo never possesses an amnion, and the urinary bladder represents the allantois. Lastly, they all undergo a metamorphose after leaving the egg. All amphibia commence their existence as water-breathing larvæ, provided with