Axolotl, the Mexican name of an amphibious reptile, described by naturalists as siredon.

This tadpole-formed reptile has the vertebras biconcave, and the body elongated and formed for swimming. The feet are four, the anterior being four-toed, the posterior five-toed; the sides of the body are marked by several small furrows, and an imperfect lateral line is continued from the gills to the tail. The head is flattened, with a rounded or truncated snout, near the end of which are the nostrils; the eyes are small, and about midway between the angle of the mouth and the nose; the tail is elongated and compressed, and tapers to a point. A thin membrane commences near the back of the head, rising gradually to the middle of the tail, and diminishing again toward the tip; underneath, it extends from behind the vent to the tip, reaching its greatest height at its anterior third. The axolotl belongs to the perennibranchiate order, or those whose gills remain through life, coexisting with rudimentary lungs; hence its respiration is always aquatic. The gill openings are large, and the gill covers are continuous beneath the throat, so as completely to separate the head from the breast.

The gills consist of four semicircular cartilaginous arches, serrated internally like those of fishes, and externally provided with fine branchial fringes, occupying thickly the lower edge of the flaps, and a few on the tip of the upper edge. The fringes are flattened, tapering, and disposed in a double row. A generic character is the presence of four external flaps, provided with respiratory fringes. There are two rows of teeth in the upper and lower jaw. There are three species described: siredon Mexicanus, Shaw; 8. maculatus, Owen; and 8. lichenoides, Baird. It is probable that other species exist, as there are many localities in Mexico, New Mexico, and Texas where "fish with legs" are common. The axolotl is about 10 inches long, of a dark brown color, with blackish spots. Great numbers are taken in the month of June from a lake about 3 m. from the city of Mexico, at an elevation of more than 8,000 feet above the level of the sea, and from water whose temperature is never below 60° F. At this time they form the principal food of the peasantry. - From the experiments of Prof. O. C. Marsh, it appears that the axolotl is the larval condition of the salaman-droid batrachian airiblystoma, usually regarded as belonging to a distinct family.

During an excursion in August, 1868, Prof. Marsh obtained from Lake Como, a small brackish sheet of water in Wyoming territory, several specimens of siredon lichenoides (Baird). On bringing them to New Haven, they went through a metamorphosis similar to that previously noticed by Dumeril in the Mexican axolotl. The first indication of the change was the appearance of dark spots on the sides of the tail, followed soon by the disappearance by absorption of the membrane along the back and below the tail. Then the external branchise began to be absorbed, and the animal came more frequently to the surface of the water for air. The spots gradually extended over the body, the external branchiae and branchial arches disappeared, and the openings on the neck were closed by the adhesion of the opercular flap. The body diminished in size; the head became more rounded above and more oval in outline; the eyes became more convex and prominent; the opening of the mouth grew larger, and the tongue considerably increased in size; changes took place in the teeth and in other parts of the structure, and finally the animal escaped from the water a true ambly-stoma, not to be distinguished from A. mavor-tium (Baird). The rapidity of these changes was greatly affected by light and temperature; under the most favorable circumstances the entire series of transformations took place in about three weeks.

It is not known that these changes occur in Lake Como, which is about 7,000 feet above the sea; and the creature no doubt breeds in its siredon or larval state. This leads to the belief that all siredons are merely larval salamanders, and to the suspicion that many other so-called perennibran-chiate batrachians, as menohranchus, siren, and proteus, may be the undeveloped young of other well known species.