From fishes the lower jaw of Archegosaurus differs in the great length or forward extension of the angular piece (30) ; but it resembles the piscine type in the simplicity of its com position. The angular piece is, however, longer in the Ganoids Amia, Polypterus, Lepidosteus,—than in other fishes ; in Lepidosiren its proportions are almost those of the Arche gosaurus ; and it offers similar proportions in the mandible of the Axolotl and Proems (fig. 65).
The teeth in Archegosaurus have the simple conical pointed shape. They are implanted in the premaxillary, maxillary, mandibular, and vomerine bone, and in a single row in each. In the short premaxillaries there are from 8 (A. Decheni) to 12 (A. latirostris); they are rather larger than the maxillary teeth. These follow in an unbroken series to beneath and beyond the orbit, and are about 30 in number ; but their in terspaces are such as would lodge double that number in the same extent of alveolar border. The vomerine teeth are in a single row, parallel with and near to the maxillary row ; one or two behind the choane are much larger than the rest, which resemble the maxillary teeth in size. The mandibular teeth extend backward to the coronoid rising, and decrease in size, the front ones being the largest. Each tooth is implanted by a simple base in a shallow cup-shaped socket, with a slightly raised border, to which the circumference of the tooth becomes anchylosed. The tooth is loosened by absorption and shed to make way for a successor. These are developed on the inner, hind, and fore part of the base of the old tooth. The teeth are usually shed alternately. They consist of osteodentine, dentine, and cement. The first substance occu pies 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 indicated by fine longitudinal, straight striae along that half of the crown. The section of the tooth at that part (see fig. 65, tooth-section) gives the same structure which is shown by a like section of a tooth of the Lepidosteus oxyurus.* The same principle of dental structure 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" Deudrodouts.
The repetition of this structure in the teeth of one of the earliest genera of Reptilia, associated with the defect of ossifi cation of the endo-skeleton and the excess of ossification in the exo-skeleton of the head and nape, instructively illustrates the true affinities and low position in the reptilian class of the so-called Arehegosauri.
Resting upon and protected by the throat-plate in the middle line, there is a longish slender bone, which must belong to the median series of the hyoid system, either basi- or uro hyal ; it is most probably homologous with the uro-hyal of Amphiuma and other Perennibranchiates. That two pairs of slender bones projected outward and backward from the median series, is shown by more than one specimen of Arche gosaurus in the British Museum. The anterior pair is the longest ; these are situated as if they had been attached, one to each side of the broad " throat-plate," which may have represented a basi-hyal. The anterior pair are homologous with the corresponding longer pair of appendages to the broad basi-hyal of Amphiuma, and are cerato-hyals. The shorter posterior pair answer to the branchi-hyals in Amphiuma and other Perennibranchs. There is no such pair in the hyoidean arch of any known Saurian.
External to the ends of the above lateral elements of the hyoid apparatus, feeble traces of arched series of bony nuclei were detected by Goldfuss, and interpreted by him as remains of partially ossified branchial arches. In all those specimens possessing them they present the outline of two or three arches in dots, or slightly curved series of dots or points. In the small relative size of these indications of branchial arches, the Archegosaurus agrees with the Amphiuma.
No doubt, in the fully-grown Archegosaurus, the lungs would be equal to the performance of the required amount of respiration ; but the retention of such traces of the embryonal water-breathing system in the adult leads to the inference that the animal must have affected a watery medium of exist ence for as great a proportion of its time as is observed to be the case in the existing perennibranchiate reptiles ; in which, notwithstanding the degree of development of the lungs, the respiratory function seems to be mainly performed by the The additional marks of affinity to fishes which the Arche gosaurus presents in its persistent notochord, cartilaginous basi-occipital, dermal ossifications on the head, and minute body-scales (fig. 65, scales), remove it further from the saurian reptiles, and exhibit it more strongly in the light of an osculant form between the Batrachians and the Ganoids.