Labyrinthodon (Gr. labyrinth, and a tooth), a gigantic fossil reptile, so named by Prof. Owen from the complex laby-rinthic structure of the teeth; the same animal had been previously called cheirotherium by Kaup, from the resemblance of its tracks to impressions of the human hand. This animal, which possesses both saurian and batrachian characters, probably most nearly resembled a gigantic frog about 10 or 12 ft. long. A historical sketch of the discoveries in connection with this reptile may be found in the "Proceedings of the Boston Society of Natural History " (vol. v., 1856, p. 298), and full details on its affinities in the " Annals and Magazine of Natural History" (vol. viii., London, 1852, pp. 305-313). Footprints and bones of the laby-rinthodon have been found in the trias of England and Germany; from an examination of the head and teeth, vertebra;, pelvis, and bones of the extremities, Prof. Owen constructed an animal intermediate between the crocodile and the frog. Pictet (Traite de paleontologie, 1853) calls it mastodonsaurus, and considers it a saurian from the presence of scutes on the skin and the form of the teeth.

The general shape of the head is frog-like, as also are the double occipital condyles, narrow palatal processes of the maxillary, the roof of the mouth, the row of small teeth across the anterior part of the palate and a longitudinal row on the palate concentric with the maxillary teeth, the lower jaw and the vertebrae, and bones of the fore limbs; on the other hand, the facial and nasal parts of the skull are crocodilian, as are the maxillary tusks, the strong transverse processes for ribs, bony dermal plates, etc. In some of the dental characters it resembles fishes. The size of the tracks varies from 4 to 12 in. in length, with five toes on each, one turned in like the human thumb; the hind foot was three or four times as large as the fore foot; there is no positive evidence that the animal had a tail; its progression seems to have been slow and awkward, the legs having been swung outward like the course of a scythe. Near each large step, and l 1/2in. before it, is a smaller one of the fore foot, the distance from pair to pair being about 14 in.

The American cheirotherium made a double series of tracks, and evidently belonged to a different genus from that of Europe.

Labyrintkodon (restored).