The femur wants both the extremities ; its shaft is sub trihedral and slightly bent, and its walls are thin and compact, including a large medullary cavity. The tibiae exhibit that remarkable compression of their distal portion which charac terizes the corresponding bone in the Batrachia ; they likewise have the longitudinal impression along the middle of the flattened surface. Were more of the skeleton of the above defined species of Labyrinthodon known, they might present differences of subgeneric value. Such differences in the forms and proportions of the skull, and in the form and relative position of the orbits, of specimens that have been discovered subsequently in the triassic sandstones of Germany, have been so interpreted.
In the Labyrinth,odon (Mastodonsaurus) Jaegeri — the largest of the species—the skull is triangular, the two condyles projecting from the middle of the base ; the sides are straight, and converge to the obtuse apex. The orbits are oval, nar rowest anteriorly, and are situated nearly midway between the fore and back part of the skull The nostrils are very small, and are as wide apart as the orbits.
Labyrinthodon (Trenzatosaurus) Braunii, Von Meyer.— The name Trematosaurus was given by Braun to a labyrin thodont reptile, in reference to the parietal foramen, at that time deemed to be peculiar to it, but now known to be com mon to all the family. The genus was founded on an unusually perfect skull discovered in the richly fossiliferous bunter sandstein of Bernburg. It is about one foot long, and, rela tively to its basal breadth, it is longer and narrower than in L. Jaeyeri, the sides converging at a more acute angle. The orbits are elliptical, situated in the middle of the skull, and wider apart than in L. Jaegeri, ; the nostrils are relatively nearer together, their interspace being only half that in the L. Jaegeri.
Labyrinthodon diagnosticus, H. von M.—In this species the skull is broader in proportion to its length than in the foregoing ; the sides are convex as they converge to the obtuse muzzle. The orbits are small, of a wide elliptical form, situated in the anterior third of the skull ; they are twice as wide apart as are the nostrils. The parietal foramen is near the occipital ridge. The remains of this species are from the upper beds of the keuper sandstone in Wirtemberg.
The Labyrinthodon (Capitosaurus) arenaceus, Munster, is distinguished by a much broader and almost truncate muzzle.
The orbits are elliptic, and situated almost wholly in the hinder third of the cranium ; their interspace is the same as that between the nostrils, which are relatively as large as in L. Braunii.
The name Zbgosaurus appears to have been applied with better grounds, by Eichwald, to a labyrinthodont reptile from the Permian cupriferous beds at Orenburg. It has the para bolic skull of L. Jaegeri and L. diagnosticus; the orbits large, and divided by an interval less than their own diameter. The temporal fosse are relatively larger, and bounded by stronger zygomatic arches, and seem not to have been roofed over by bone. The dentition is strictly labyrinthodont.
Odontosaurus Voltzii is a genus and species founded by Von Meyer on a portion of a lower jaw, containing fifty teeth lodged in rather a deep groove, but apparently presenting the labyrinthic structure. The specimen is from the bunter sand stone of Soultz-les-Bains.
Xestorrliytias Perrini.—By this name M. von Meyer would indicate certain flat cranial bones, sculptured like those of * This generic term has been applied to another fossil by Eichwald.
Labyrinthodon, but with a peculiarly polished ganoid-like surface, from the muschelkalk of Lundville.
In all the foregoing forms of Labyrinthodonts, represented by complete crania, with the exception perhaps of Zygosaurus, the supplemental osseous plates roofing over the temporal fosse are present, as in Archegosaurus, viz., the " post-orbital " and the " super-squamosal " bones. In all of them the occipital condyles are distinct, forming a pair ; and in all the voiner is divided and bears teeth. The structure and disposition of the entire dental system is strictly labyrinthodont.
The relation of these remarkable reptiles to the saurian order has been advocated to be one of close and true affinity, chiefly on the character of the extent of ossification of the skull, and of the outward sculpturing of the cranial bones. But the true nature of some of these bones appears to have been overlooked, and the glance of research for analogous structures has been too exclusively upward. If directed down ward from the Labyrinthodonts to the Archegosauri and certain ganoid fishes, it suggests other conclusions.