[744] Geol. Trans. 2d ser. vol. vi. pl. xxi. p. 203; and Brit. Foss. Mam. p. 419, &c.
The Paloplotherium, an allied genus, from Hordwell Cliff, is described in Geol. Journ. vol. iv. p. 103.
The other large fossil Pachyderms, belonging to the two existing genera of Rhinoceros and Hippopotamus, are found very extensively distributed in alluvial debris, in the ossiferous breccia of caverns, and in other pleistocene deposits; and their remains are frequently dug up in the superficial marls, clays, gravel, and sand of England. As the teeth of these animals will occasionally be met with by the collector, a brief explanation of their form and structure may be useful.
Lign. 257. Hyopotamus.
Incisor Teeth: nat. size. Upper Eocene. Isle of Wight.
| Fig. | 1.— | Inner surface of an incisor of Hyopotamus. |
| 3.— | Lateral view of an upper incisor of Hyopotamus. | |
| 4.— | Outer aspect of the crown of the same tooth. | |
| 5.— | Inner aspect of ditto. | |
| 2.— | Lateral view of the upper incisor of recent Hog (Sus scrofa). |
TEETH OF MAMMALIA.
Teeth of Mammalia.[745]—The organization of the teeth in the herbivorous mammalia essentially consists in the adaptation of the three elements of dental structure to the peculiar conditions required by the habits and economy of the different species. Thus, in the Elephant ([Lign. 259], 260), Horse ([Lign. 263]), &c., the dentine, cement, and enamel are disposed in vertical plates more or less inflected, the enamel and cement penetrating the body of the tooth, and embracing corresponding processes of dentine; an arrangement by which a grinding surface, composed of three substances of unequal densities, is produced and maintained in every state of detrition (Owen). But these teeth do not possess the symmetrical and complicated structure observable in those of many of the reptiles and fishes we have previously investigated. In the carnivorous mammalia, the enamel constitutes an external shell or case, investing the body of dentine and presenting sharp cusps or trenchant ridges, adapted for the laceration of flesh, as in the Tiger, or modified so as to form instruments for snapping and crushing bones, as in the teeth of the Hyæna. In the Mastodon, the crown of the tooth, when first emerged from the gum, presents a series of strong conical eminences ([Lign. 254]), that become worn down by use, at first into disks (Ly. p. 157), which, by further detrition, coalesce. The tooth of the Elephant ([Lign. 259 and 260]), on the contrary, consists of vertical plates of dentine, with an immediate investment of enamel, over which there is an external layer of cement that binds together the entire series of plates, often amounting to twenty or more; the horizontal surface produced by the detrition of such a structure, gives rise to the well-known grinding surface of the molars of the elephant ([Lign. 259], 260; Wond. pp. 143 and 160; Ly. p. 159; Owens Brit. Foss. Mam. figs. 88-90, &c.). Detached plates of the teeth of Elephants, particularly of those which belong to the back part of the posterior grinder, and have not come into use, are puzzling to the inexperienced collector of fossil remains; and the first indication I obtained of the existence of the remains of fossil Elephants in Brighton Cliffs (Wond. p. 150), was from a mass of this kind, dug up in sinking a well in Dorset Gardens, and sent to me as a "petrified cauliflower."