158 report— 1859. 



mary cartilage internally, and ossification to have been chiefly active at the 

 surface, where, a< in the combined dermo-neural ossifications of the skull 

 in the sturgeons and salamandroid fishes, e. g. Polypterus, Amia, Lepidosteus, 

 these ossifications have started from centres more numerous than those of 

 the true vertebral system in the skull of Saurian reptiles. 



The teeth are usually shed alternately; they consist of osteo-dentine, 

 dentine, and cement. The first substance occupies the centre, the last 

 covers the superficies of the tooth, but is introduced into its substance by 

 many concentric folds extending along the basal half. These folds are indi- 

 cated by fine longitudinal straight stria? along that half of the crown. The 

 section of the tooth at that part gives the same structure which is shown by 

 a like section of a tooth of the Lepidosteus oxyurus*. 



The same principle of dental composition is exemplified in the teeth of 

 most of the ganoid fishes of the Carboniferous and Devonian systems, and is 

 carried out to a great and beautiful degree of complication in the Old-red 

 Dendrodonts. 



The repetition of the same principle of dental structure in one of the ear- 

 liest genera of Reptilia, associated with the defect of ossification of the endo- 

 skeleton and the excess of ossification in the exoskeleton of the head, deci- 

 sively illustrate the true affinities and low position in the Reptilian class of 

 the so-called Archegosauri. 



For other details of the peculiar and interesting structure of the animals 

 representing the earliest or oldest known order of Reptiles, the Reporter 

 would refer to his article " Palaeontology" in the ' Encyclopaedia Britannica,' 

 and to the works by Goldfuss and Von Meyer, above cited. 



Order II. Labyrinthodontia. 



This name, from \afivpivdos, a labyrinth, and olovs, a tooth, refers to the 

 complex structure characterizing the teeth, in the several genera of the order ; 

 in which, also, the head is defended, as in the Ganocephala, by a continuous 

 casque of externally sculptured and usually hard and polished osseous plates, 

 including the supplementary ' postorbital ' and ' supratemporal ' bones, but 

 leaving a ' foramen parietalef .' There are two occipital condyles. The 

 vomer is divided and dentigerous. There are two external nostrils. The ver- 

 tebral centra, as well as arches, are ossified, and are biconcave. The pleur- 

 apophyses of the trunk are long and bent. The teeth are rendered complex, 

 at least at the basal part of the crown, by undulations and side-branches of 

 the converging folds of cement ; whence the name of the order. 



The reptiles presenting the above characters have been divided, according 

 to minor modifications exemplified by the form and proportions of the skull, 

 by the relative position and size of the orbital, nasal, and temporal cavities, 

 and by dermal characters, into several genera ; as, e. g. Mastodonsaurus, Ani~ 

 sopus, Trematosaurus, Metopias, Capitosaurus, Zygosaurus, Xestorrhytias, 

 &c. 



The relation of these remarkable reptiles to the Saurian order has been 

 advocated as being 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 gaze of research for analogous structures has been 

 too exclusively upward. If directed downward from the Labyrinthodontia 

 to the Ganocephala, and to certain ganoid fishes, it suggests other conclusions 

 which I have developed in my article " Palaeontology " above referred to. 



* Wyman, ' American Journal of the Natural Sciences,' October, 1843. 

 t The corresponding vacuity is larger in some ganoid fishes. 



