Lepidosiren, a vertebrated animal, possessing characters of both fishes and reptiles, and alternately referred by naturalists to one or the other of these classes. This animal was discovered by Dr. Natterer in the river Amazon in 1837, and was referred by him and Fitz-inger to this genus, considered by them as belonging to the fish-like or perennibranchiate reptiles. ' Prof. Owen (" Linnaean Transactions," vol. xviii., and "Proceedings of the Linna3an Society," April 2, 1839) had recorded this same paradoxical animal, in his MS. catalogue of the museum of the college of surgeons (1837), as a new genus of abdominal malacop-terygian fishes, under the name of protopterus; he afterward made this family of sirenidae the type of a distinct order of fishes, the protop-teri (the sirenoidei of Muller); he referred it to fishes on account of its scaly covering and of its nostrils not communicating with the mouth, and to the abdominal malacopterygians from its soft and rudimentary fins, indicating a transition from the abdominal to the apodal families, and for various other anatomical reasons.

The skeleton is partly osseous, partly cartilaginous; the body is fish-like in form, and covered with cycloid scales; the pectorals and ventrals are mere jointed flexible rays; the bodies of the vertebras remain in the embryonic state of a continuous chondro-gelati-nous cord, though many other parts of the skeleton are well ossified. This transitional state between the embryonic condition of ossification of the vertebral centre and that of ordinary bony fishes, was common in the ganoid fishes, not one of which in the Silurian or Devonian epochs, according to Agassiz, had a vertebral centrum. There are 36 pairs of ribs, encompassing about one sixth of the abdominal cavity; immediately in front of the pectorals there is a vertical branchial opening; on the intermaxillary bones are two long, slightly curved, slender, acute teeth, on the upper jaw on each side a dental plate divided into three cutting lobes, and on the lower jaw a similar single plate whose lobes fit into the intervals of the upper, fitted for minute division of food; the tongue is well developed, the pharynx with a small valve-protected opening, the gullet short and narrow, the stomach thick, simple, and straight, the liver of good size with gall bladder, and the straight intestine with an internal spiral fold; there is neither pancreas nor spleen.

The respiratory organs consist of branchiae, with a double elongated air bladder resembling the cellular lungs of a reptile; the branchial sac is large, and the gills are supported on four arches on each side, two additional arches offering no trace of gills, there being five intervals for the passage of water into the pharynx; the nasal cavities open into the mouth (this is denied by Owen), and the laryngeal opening leads to the honeycombed air bladders or lungs, which are behind the kidneys and internal reproductive organs; the kidneys are long and narrow, the ureters and the genital ducts opening into the cloaca; the heart, in a strong pericardium, has a single ventricle, a single imperfectly divided auricle, and an arterial bulb, a large part of the blood in the adult being sent to the air bladders for purification. The eyes are small and adherent to the skin, which passes over them without forming any projection, and the lens is small and spherical; there is no trace of tympanic cavity nor Eustachian tube.

The scaly covering, soft fin rays, characters of the spinal canal and cord, mucous ducts and lateral line, peculiarities of the cranial and jaw bones, intestinal spiral valve, absence of spleen and pancreas, single auricle, the nasal sacs opening only externally (the last denied by many), and the articulation of the scapular arch to the occiput, prove, according to Owen, that the lepidosiren is a fish, and not a batrachian, forming a connecting link between cartilaginous and soft-rayed fishes, and coming in this class the nearest to the perennibranchiate reptiles. The L. paradoxa (Natterer), from the morasses of the river Amazon in Brazil, attains a length of about 3 ft.; when the water dries up, they plunge under the mud; the food is said to consist of vegetable matters. The L. annec-tens (Owen), from the river Gambia and also the Mozambique coast, is a smaller species, rarely more than 2 ft. long. In the "Proceedings of the Zoological Society of London " for Nov. 11, 1856, Mr. J. E. Gray advocates the batrachian nature of the lepidosiren, three specimens of which were brought alive from Africa, enclosed in balls of hardened clay in which they remained torpid during the eight months of the dry season; they were on exhibition at the crystal palace at Sydenham for a considerable time, and one for several months.

From the account there given it appears that this animal can move with considerable rapidity forward and upward by means of its tail, which is surrounded by a membranous expansion like a confluent dorsal and anal fin. The pectoral limbs are elongated and margined behind with a narrow membrane, the ventral having a similar edging in the middle of the outer side; they are very mobile and flexible, and are used more like feet than fins, supporting the body about two inches from the bottom, and also serving to direct its motions; the two processes on each side over the pectorals, considered as external gills by some, he regards as a portion of the anterior limb, as they possess no peculiar vascular structure; the movements are much more like those of the water salamanders than of eel-shaped fishes. The mucous pores on the head and the lateral line are common to fishes and some batrachians; the small, circular pupil is black, and the narrow iris golden; the mouth is firmly closed by the overhanging upper lip, except in front, where the water is admitted to open external nostrils on the middle of the under side of the upper lip; the lips close behind, so that water cannot pass into the mouth under these circumstances except through the nostrils; the internal openings of the nostrils are just behind the edge of the closed lips, and through them the animal breathes water in the quiescent state, passing it out at the gill aperture in front of the pectorals; it also introduces water to the gills through the widely extended mouth.

As if this were not sufficient for respiration, it occasionally rises to the surface and takes in air by the open mouth, and swallows it into the sacculated lungs, a few bubbles generally escaping from the gill aperture. These internal nostrils were noticed by Bischoff in the L. paradoxa. It thus appears that the lepidosiren, or mud fish, breathes by both gills and lungs, taking in water by the nostrils, and respiring air like batrachians and water like fishes, constituting as near an approach to an amphibious animal as is known to exist; it probably can no more live on air alone than can the menobranchus or fish lizard of the North American lakes. They are abundant in the rice fields, which are under water for more than half the year; the natives dig them out of the mud toward the end of the dry season, and consider them a delicacy. The mud cocoons in which they were carried to England had each a small opening at the end where the nose of the animal is placed; as developed at the crystal palace, they were very thin and 9 in. long when they left the cocoon, but began to feed at once on worms, small frogs, fish, and raw meat, attacking each other with fury, and one at last killing and half devouring another; in three months they attained a length of 18 in.

The movements, as in the meno-branchus, are generally sluggish, but they are capable of very rapid motion; the food seems to be detected as much by scent as by sight. While in the cocoons they are in a state of hibernation, the blood being sufficiently purified by the arterial trunks distributed to the air bladders. The color of the L. annectens is a mixed tint of dark olive-green and brown, lighter below, with irregular dark spots as large as the scales chiefly confined to the tail, and the mucous pores and lateral line black. The anus does not open on the median line of the body. With such contradictory opinions as to the position of this animal, possessing characters both of the fish and the batrachian, it would seem to belong to a distinct order (dipnoi), forming one of the most interesting links between the ichthyoid batrachians and the cartilaginous fishes.