Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/222

 218 appear for the most part related to genera now common in tropical seas, but are all of extinct species.

As the family of Sharks is one of the most universally diffused and most voracious among modern Fishes, so there is no period in geological history in which many of its forms did not prevail. Geologists are familiar with the occurrence of various kinds of large, and beautifully enamelled teeth, some of them resembling the external form of a contracted leech, (Pl. 27e, and 27f): these are commonly described by the name of Palate bones, or Palates. As these teeth are usually insulated, there is little evidence to indicate from what animals they have been derived.

In the same strata with them are found large bony Spines, armed on one side with prickles, resembling hooked teeth, (see Pl. 27d. C. 3. a.) These were long considered to be jaws, and true teeth; more recently they have been ascertained to be dorsal spines of Fishes, and from their supposed defensive office, like those of the genus Balistes and Siluris, have been named Ichthyodorulites.

M. Agassiz has at length referred all these bodies to extinct genera in the great family of Sharks, a family which he separates into three sub-families, each containing forms peculiar to certain geological epochs, and which change simultaneously with the other great changes in fossil remains.

The first and oldest sub-family, Cestracionts, beginning with the Transition strata, appears in every subsequent formation, till the commencement of the Tertiary, and has only one living representative, viz. the Cestracion Philippi, or Port Jackson Shark. (Pl. 1. Fig. 18.) The second