[524] Viz. Ganodus, Ischyodus, Edaphodon, Elasmodus, and Psaliodus. See Quart. Journ. Geol. Soc. vol. iii. p. 35; and Dixon’s Foss. Sussex.
In some species the external vertical wall of the plate is formed of hard dentine, resembling enamel; in others the dentine is disposed in isolated ramifications, producing a dendritical appearance; the modifications of this structure occasion the differences observable in the dental plates of the various species. In some, compact dentine with parallel canals constitutes the mass of the tooth; in others, the squamous dentine with ramifying tubes prevails.
I have figured the right upper and lower mandibles of the type named Edaphodon (pavement-tooth), in which there are three teeth or dental tubercles on each ramus of both jaws, [Lign. 191]: the lower mandible is produced anteriorly into a falciform beak:[525] the articulating surface of the symphysis (2 s) is broad at the base, and contracts gradually forward until the margins meet at the apex. In Ischyodus the lower jaw is deeper, less produced in front, and the margins of the symphysis are parallel until abruptly truncated at the extremities: the upper jaw has four tubercles on each side.
[525] Hence M. Agassiz proposed the name of Psittacodon (parrot-tooth) for this genus of Chimæroids.
The upper jaw in Elasmodus has but three tubercles, as in Edaphodon, but the dentine of which they are composed is confluent, being rolled round like a scroll in the substance of the bone, one edge forming the margin of the tooth, and the other being buried deep in its centre.
The dorsal fin-ray or spine of the Edaphodon is laterally compressed, with the posterior margin grooved, and the edges armed with fine teeth: I have a specimen of the spine, with a pair of inferior mandibles of the same individual, imbedded in a block of chalk from Kent; by favour of Mrs. Smith, of Tunbridge Wells.
Lign. 192. Fossil Teeth of Sharks.
| Fig. | 1.— | Hybodus medius. Lias. Lyme Regis. |
| 2.— | Hybodus raricostatus. Lias. Bristol. | |
| 3.— | Carcharodon productus. Tert. Malta. | |
| 4.— | Hemipristis serra (fragment). Chalk. Ratisbon. | |
| 5.— | Otodus Obliquus. Tert. Sheppey. |