ON THE STRUCTURE OF THE LABYRINTHODONTS. 239 



chiderpeton the vomerine plates are armed with clustered teeth, resembling 

 the aggregated teeth of Siren and Siredon. The mandible bears a rcw of 

 teeth, which may be continuous, or interrupted by large tusks and depressions. 

 A pair of tusks is frequently found near the anterior end of the rami. In 

 Labyrinthodon, Trematosaurus, and some other genera a short inner series 

 of large teeth is found near the symphysial end of the mandible. Among 

 recent Amphibia a double row of mandibular teeth occurs in Epicrium and 

 Siredon : it is present in many fishes. 



Most commonly a number of teeth are represented only by gaps, or scars 

 upon the alveolar border ; the vacant places frequently alternate with the 

 standing teeth, rendering it probable that about half the teeth were normally 

 efficient at the same time, and that they were replaced alternately. The 

 substitution of the palatal teeth was less regular : new teeth appear to have 

 been usually developed upon vacant spaces ; but in some instances the 

 successional tooth appears in front, behind, or to the inner side of its 

 l^redecessor. 



Vertebral Column. — The following general features of the vertebral column 

 of Labyrinthodonts may be noted : — 



a. The number of vertebras is large. 



b. There are at least two kinds of vertebrae — thoracic and caudal. 



c. The centra are biconcave. 



d. A superior arch and spine are present in all the vertebrae which are ac- 

 curately known. 



e. Inferior arches are present in the caudal region. 



/. Where zygapophjses are present, the anterior look more or less inwards 

 and generally upwards also. 



g. The spinal foramen is much contracted. 



The chief variations which occur in the corresponding vertebrae of different 

 species are these : — 



The centra vary greatly with respect to their degree of ossification. In 

 Archegosauriis, for example, the notochord is persistent, and the only osseous 

 parts of the vertebrae are the superior arches, superior spinous processes, 

 trausverse processes (proceeding from the laminae of the superior arches), 

 inferior arches, inferior spinous processes, and lateral wedge-bones ("seitliche 

 KeUe" of Von Meyer = "interneural and interhaemal pieces"?). It has 

 been suggested by Professor Huxley * that the inferior arches and lateral 

 wedge-bones may represent osseous rings, like those which remain of the 

 centra of Megalichihys, and that " they have broken up into the separate 

 pieces described by Von Meyer in the process of fossilization." In Mastodon- 

 sauria, on the contrary, and most of the undoubted Carboniferous Labyrin- 

 thodonts, the centra are well ossified. In Loxomma and Anihracosaurus a 

 small notochordal foramen is apparently persistent. A neuro-central suture 

 appears to have been permanently present in some, if not in all. 



The centra of the Carboniferous species are usually discoidal, the antero- 

 posterior length being small ; but the vertebrae of Ophiderpeton and Lepter- 

 peton, as well as those of Labyrinthodon f and some of the Microsauria of 

 Dawson have hourglass- shaped centra of considerable longitudinal extent. 



There are usually two articular facets for the ribs, both situate on the 

 neural transverse process. In Masfodonsaurtcs, however, the lower facet is 

 continuous with the centrum J ; and an example of the vertebral column of 



* Quart. Journ. Geol. Soc. vol. six. p. 67 (note) (1863). 



t L. leptoc/nafh(s (Owen, ' Trans. Geol. Soc' vol. vi. t. xlv. figs. 5-8). 



% PalJioTitologie WiirtcmborgB, t. iv. fig. 8, and p. .58 (1844). 



