In the structure of these teeth we find a combination of mechanical contrivances ana logous to those which are adopted in the construction of the knife, the sabre, and the saw. When first protruded above the gum, the apex of each tooth presented a double cutting edge of serrated enamel. In this stage, its position and line of action were nearly vertical, and its form, like that of the two edged point of a sabre, cutting equally on each side. As the tooth advanced in growth it became curved backwards in the form of a pruning-knife, and the edge of serrated enamel was continued downwards to the base of the inner and cutting side of the tooth, whilst on the outer side a similar edge descended but a short distance from the point, and the convex portion of the tooth became blunt and thick, as the back of a knife is made thick for the purpose of producing strength. The strength of the tooth was further in creased by the expansion of its side. Had the serrature continued along the whole of the blunt and convex portion of the tooth, it would in this position have possessed no useful cutting power ; it ceased precisely at the point beyond which it could no longer be effective. In a tooth thus formed for cutting along its concave edge, each movement of the jaw combined the power of the knife and saw ; whilst the apex, in making the first incision, acted like the two-edged point of a sabre. The backward curvature of the full-grown teeth enabled them to retain, like barbs, the prey which they had penetrated. In these adaptations we see contrivances which human ingenuity has also adopted in the preparation of various instruments of art." The teeth of the Megalosaur consist of a central body of dentine, with an investment of enamel upon the crown, and of cement over all, but thickest upon the fang. The marginal serrations are formed almost entirely by the enamel, and when slightly magnified are seen to be rounded, and separated by slight basal grooves ; the smooth and polished enamel upon the sides of the crown presents a finely wrinkled appearance; the remains of the pulp are converted into a coarse bone in the completely formed tooth.
Enaliosaurs. — The teeth of the Ichthyo sauri have a simple, more or less acutely conical form, with a long and, usually, ex panded or ventricose base, or implanted fang. They are confined to the intermaxillary, max illary, and premandibular bones, in which they are arranged in a pretty close and un interrupted series, and are of nearly equal size. They consist of a body of unvascular dentine, invested at the base by a thick layer of cement, and at the crown by a layer of enamel, which is itself covered by a very thin coat of cement; the pulp-cavity is more or less occupied in fully-formed teeth by a coarse bone. The external surface of the tooth is marked by the longitudinal impressions and ridges, but the teeth vary both as to outward sculpturing and general form in the different species.* The chief peculiarity of the dental system of the Ichthyosaur is the mode of the im plantation of the teeth ; instead of being an chylosed to the bottom and side of a con tinuous shallow groove, as in most Lacertians, or implanted in distinct sockets, as in the The codon, Megalosaur,• or Pterodactyle, they are lodged loosely in a long and deep continuous furrow, and retained by slight ridges between the teeth, along the sides and bottom of the furrow, and by the gum and organised mem branes continued into the groove and upon the base of the teeth.
The germs of the new teeth are developed at the inner side of the base of the old ones.
Crocodilia. — The best and most readily recognisable characters by which the existing Crocodilians are grouped in appropriate genera, are derived from modifications of the dental system.
In the Caimans (genus Alligator) the teeth 18-18 vary in number from 18--18 to 22-22: the fourth tooth of the lower jaw, or canine, is received into a cavity of the palatal surface of the upper jaw, where it is concealed when the mouth is shut. In old individuals the upper jaw is perforated by these large in ferior canines, and the fossm are converted into foramina.
In the Crocodiles (genus Crocodiles) the first tooth in the lower jaw perforates the palatal process of the premaxillary bone when the mouth is closed ; the fourth tooth in the lower jaw is received into a notch exca vated in the side of the alveolar border of the upper jaw, and is visible externally when the mouth is closed.
In the two preceding genera the alveolar borders of the jaw have an uneven or wavy contour, and the teeth are of an unequal size.
In the Gavials (genus Gavielis) the teeth are nearly equal in size and similar in form in both jaws, and the first as well as the fourth tooth in the lower jaw, passes into a groove in the margin of the upper jaw when the mouth is closed.
In the alligators and crocodiles the teeth are more unequal in site, and less regular in arrangement, and more diversified in form than in the Gavials : witness the strong thick conical laniary teeth as contrasted with the blunt mammillate summits of the posterior teeth in the alligator (fig. 573.). The teeth of the Gavial are subequal, most of them pre sent the form of crown, shown in fig. 572., long, slender, pointed, subcompressed from before backwards, with a trenchant edge on the right and left sides, between which a few faint longitudinal ridges traverse the basal part of the enamelled crown.
Amongst the remains of Crocodilians which are scattered through the Tilgate strata, the most common ones are detached teeth, from the difference observable in the form of which, Dr. Mantel! has observed, that " they appear referable to two kinds, the one belonging to that division of crocodiles with long slender muzzles, named Gavial, the other to a species of Crocodile, properly so-called, and resembling a fossil species found at Caen."# Dr. Mantell has obligingly communicated to me figures of well-preserved specimens of both the forms of teeth alluded to, the exact ness of which I have recognised by a com parison with the specimens themselves in the British Museum.