A symmetrical bone, resembling the epistemum of the Ich thyosaurus was associated with the preceding remains. It consists of a stem or middle, which gradually thickens to the upper end, where cross pieces are given of at right angles to the stem, and support on each a pretty deep and wide groove indicating strongly the presence of clavicles, and thus pointing out another distinction from crocodiles, in which clavicles are wanting. Most Batrachians possess these bones.
The modifications of the jaws, and more especially those of the bony palate of the Labyrinthodon leptognathus, prove the fossil to have been essentially Batrachian, but with affinities to the higher Sauria, leading, in the form of skull and the sculp turing of the cranial bones, to the crocodilian group, in the collocation of the larger fangs at the anterior extremities of the jaws to the Plesiosaurus, and in one part of the dental structure, in the form of the episternum, and the biconcave vertebrae, to the Ichthyosaurus. Another marked peculiarity in this fossil is the anchylosis of the base of the teeth to dis tinct and shallow sockets, by which it is made to resemble the Sphyrxna and certain other fishes. From the absence of any trace of excavation at the inner side of the base of the func tional teeth, or of alveoli of reserve for the successional teeth, it may be concluded that the teeth were reproduced, as in the lower Batrachians and in many fishes, in the soft mucous membrane which covered the alveolar margin, and that they subsequently became fixed to the bone by anchylosis, as in the pike and Lophius.
Labyrinthodon paehygnathus.—The remains of this species, which have been obtained, consist of portions of the lower and upper jaws, an anterior frontal bone, a fractured humerus, an ilium with a great part of the acetabulum, the head of a femur, and two ungual phalanges. A portion, nine and a half inches long, of a right ramus of a lower jaw, in addition to the charac ters common to it and the fragment of the lower jaw of the L. leptognathus, in the structure of the angular and dentary pieces, shows that the outer wall of the alveolar process is not higher than the inner, as in frogs and toads, the salamanders and menopome, in all of which the base of the teeth is anchy losed to the inner side of an external alveolar plate. The smaller serial teeth are about forty in number, and gradually diminish in size as they approach both ends, but chiefly so towards the anterior part of the jaw. The sockets are close together, and the alternate ones are empty. The great laniary teeth were apparently three in each symphysis, and the length of the largest was one inch and a half, The base of each tooth is anchylosed to the bottom of its socket, as in scomberoid and sauroid fishes ; but the Labyrinthodon possesses a still more ichthyic character in the continuation of a row of small teeth anterior and external to the two or three larger tusks. The
premaxillary bone presents the same peculiar modification as in the higher organized Batrachia, the palatal process of the premaxillary extending beyond the outer plate both externally and, though in a less degree, internally, where it forms part of the boundary of the anterior palatal foramen, whence the outer plate rises in the form of a compressed process from a longitudinal tract in the upper part of the palatal process ; it is here broken off near its margin, and the fractured surface gives the breadth of the base of the outer plate, stamping the fossil with a Batrachian character conspicuous above all the saurian modifications by which the essential nature of the fossil appears at first sight to be masked.
In the pre-frontal bone there are indications of crocodilian structure. Its superior surface is slightly convex and pitted with irregular impressions ; and from its posterior and outer part it sends downwards a broad and slightly concave process, which appears to be the anterior boundary of the orbit. This process presents near its upper margin a deep pit, from which a groove is continued forwards ; and in the corresponding orbital plate of the crocodile there is a similar but smaller foramen.
From these remains of the cranium of the L. pachygnathus it is evident that the facial or maxillary part of the skull was formed in the main after the crocodilian type, but with well marked batrachian modifications in the prema.xillary and inferior maxillary bones. The most important fact which they show is, that this sauroid Batrachian had subterminal nostrils, leading to a wide and shallow nasal cavity, separated by a broad and almost continuous palatal flooring from the cavity of the mouth ; indicating, with their horizontal posi tion, that their posterior apertures were placed behind the anterior or external nostrils, whereas in the air-breathing Batrachia the nasal meatus is short and vertical, and the inter nal apertures pierce the anterior part of the palate, suitable to their mode of breathing by deglutition. It may be inferred, therefore, that the apparatus for breathing by inspiration must have been present in the Labyrinthodon as in the crocodile ; and that the skeleton of the Labyrinthodon will be found to be provided with well-developed costal ribs, and not, as in most of the existing Batrachians, with merely rudimentary styles. Since the essential condition of this defective state of the ribs of Batrachians is well known to be their fish-like mode of generation and necessary distension of the abdomen, it is probable that the generative economy of the Labyrinthodonts, in which the more complete ribs would prevent the excessive enlargement of the ovaria and oviducts, may have been similar to that of saurian reptiles.