Hybodus. [Lign. 192], figs. 1, 2. (Bd. pl. xxviid.)—Intermediate between the obtuse crushing teeth of the Sharks previously described, and those sharp, angular, pointed, dental organs of the Squaloids, are those of the fishes which M. Agassiz has arranged in a sub-family or group termed Hybodonts; the teeth of which are characterised by their transversely elongated form, and the series of subacute, compressed, conical cusps or points, which compose the crown. The median cone is the principal, the lateral points being shorter and smaller, as in [Lign. 192], fig. 2; in some species the difference between the median and lateral cones is greater, in others less, as in fig. 1. These cusps have a coating of dense enamel, which is plicated longitudinally on both faces. The base, which almost equals the crown in size, is composed of a coarse osseous substance. The internal structure of the crown differs from that of the Cestracionts, in having no principal pulp-cavity, and in being chiefly composed of dendritical dentine, with reticulated medullary canals. The form and organization of these teeth show them to have been instruments for cutting and tearing food. The Hybodonts, as we have already stated ([p. 581]), possessed two spinous dorsal fins; in their habits and economy they probably did not differ from the ordinary Sharks. Teeth and spines of this genus are common in the Trias, Lias, Oolite, and Walden, and occur in the Green Sand and Chalk. There are several species of teeth and fins in the strata of Tilgate Forest (Foss. Til. For. pl. x.). In general the teeth are found detached, but occasionally they occur in their natural position, adhering to the mineralized cartilaginous jaws (Petrif. Lign. 97); as in the beautiful fossil figured Bd. pl. xxvii.d; and in the fine specimen of H. basanus, from the Isle of Wight, figured in the Geol. Soc. Journ. vol. pl. iv. There are several related genera, founded on the situation, form, and division of the principal cusps of the teeth.
Sharks with Cutting Teeth.—The jaws of the common squaloid fishes, as the Lamna (Porbeagle) and Carcharias (Great White Shark), are so common in collections of natural history, as to render a description unnecessary. The numerous vertical rows of angular, laterally compressed, pointed teeth, with sharp or serrated edges—in some species consisting of a simple trenchant cusp, in others with small lateral teeth, or denticles, at the base, are characters with which all are familiar. Fossil teeth of this form are extremely abundant in the Tertiary and Cretaceous deposits; and are commonly in a beautiful state of preservation. The genera of these fossil teeth are founded on the solidity or hollow structure of the cusps, their possessing cutting or serrated edges, and the presence or absence of lateral denticles. But the last character cannot in every instance be relied upon, for some recent Sharks have rows of teeth both with and without denticles.
Carcharodon productus. [Lign. 192], fig 3.—The genus Carcharias comprises the large Sharks with cutting triangular teeth, crenated (notched) on their margins, and having a broad base. In Carcharodon, the teeth differ from those of Carcharias in being solid in the centre, while in the latter they are hollow; but in both genera the teeth exhibit the same reticulated structure of medullary and calcigerous tubes. The White Shark and other large species belong to these genera; some of which are upwards of forty feet in length. But even these colossal fishes must have been far surpassed in magnitude by the extinct species of the Tertiary deposits, if the teeth afford a scale of proportions; for some of the fossil teeth from Malta and the United States are six inches long, and five wide at the base;[526] being twice the size of the teeth in the largest living species. The specimen figured in illustration, [Lign. 192], fig. 3, is of a small size.
[526] For instance, some of the Maryland specimens of Carcharodon megalodon. See an admirable memoir on the Fossil Squalidæ of the United States, by Dr. R. W. Gibbes, Journ. Acad. Nat. Science, Philadelphia. At the meeting of the British Association in 1851, J. S. Bowerbank, Esq. F.R.S. read some interesting observations on the comparison of these large fossil fishes with the recent Carcharias glaucus of Australia. See also Owen, Cyclop. Anat. Art. Teeth.
Hemipristis serra. [Lign. 192], fig 4.—The fossil teeth of this genus are distinguished by serrated edges, that do not extend to the summit, which is a sharp angular point; as in the fossil represented.
Lign. 193. Fossil Teeth of Sharks. Chalk. Lewes.
| Fig. | 1.— | Corax pristodontus. |
| 2.— | Lamna crassidens. | |
| 3.— | Notidanus microdon. | |
| 4.— | Ptychodus polygurus; seen laterally. | |
| 5.— | Ptychodus polygurus; viewed from above. | |
| 6.— | Lamna elegans. |