The exterior of the long tusk is marked by spiral ridges, which wind from within forward, upward, and to the left. About fourteen

220

Base of skull of male Narwhal, with a section of the Tusk.

inches is implanted in the socket; it tapers gradually from the base to the apex. The pulp-cavity, as shown in the longitudinal section of the tusk, in fig. 220, is continued nearly to the extreme point, but is of variable width at the base it forms a short and wide cone; it is then continued forward, as a narrow canal, along the centre of the implanted part of the tooth, beyond which the cavity again expands to a width equalling half the diameter of the tooth; and finally, but gradually, contracts to a linear fissure near the apex. Thus, the most solid and weighty part of the tooth is that which is implanted in the jaw, and nearest the centre of support, whilst the long projecting part is kept as light as might be compatible with the uses of the tusk as a weapon of attack and defence. The portion of pulp, in which the process of the calcification has been arrested, receives its vessels and nerves by the fissure continued from the basal expansion of the pulp-cavity. In a few instances, both tusks have been seen to project from the jaw.

In Delphinus griseus the dentition of the upper jaw is transitory, as in Hyperoödon, but at least six pairs of teeth rise above the gum and acquire a full development at the forepart of the lower jaw. The crowns of these teeth soon become obtuse, and their duration is limited: aged individuals of this species have been taken with the dentition reduced to

two teeth in the lower jaw.

The outward and visible dentition of the great Sperm-whale or Cachalot (Physeter macrocephalus) is confined to the lower jaw. The series consists in each ramus of about twenty-seven teeth. In the young they are conical and pointed; usage renders them obtuse, whilst progressive growth expands and elongates the base into a fang, which then contracts, and is finally solidified and terminated obtusely. The teeth are separated by intervals as broad as themselves. The mode of implantation is intermediate between that of the teeth of the Ichthyosaurus, and of those of Delphinus. They are lodged in a wide and moderately deep groove, imperfectly divided into sockets, the septa of which reach only about half-way from the bottom of the groove. These sockets are both too wide and too shallow to retain the teeth independently of the soft parts, so that it commonly happens, when the dense semi-ligamentous gum dries upon the bone, and is stripped off in that state, that it brings away with it the whole series of the teeth like a row of wedges half-driven through a strip of board. A firmer implantation would seem unnecessary for teeth which have no opponents to strike against, but which enter depressions in the opposite gum when the mouth is closed. That gum, however, conceals a few persistent specimens of the primitive fœtal series of teeth; these are always much smaller and more curved than the functional teeth of the lower jaw, of which a section is given in fig. 239, vol. i. p. 362. In the small snub-nosed Cachalot (Physeter simus) the first tooth of this series is exposed in the front of the upper jaw.1

The first-formed extremity of the tooth in the young Cachalot is tipped with enamel: when the summit of the crown has been abraded, the tooth consists of a hollow cone of dentine, ib. d, coated by cement, c, and more or less filled up by the ossified pulp, o. Irregular masses of this fourth substance have been found loose in the pulp-cavity of large teeth. The external cement is thickest at the junction of the crown and base, which are not divided by a neck.

The permanent or mature dentition of the Beluga (Delphinus leucas, Pall.), though scanty, is more normal than in the Physeter, nine functional teeth being retained on each side of the upper jaw, and eight in each ramus of the lower jaw. They present the form of straight subcompressed obtuse cones.

The most formidable dentition is that of the predaceous Grampus (Phocæna orca), whose laniariform teeth are as large in proportion to the length of the jaws as in the crocodile; they are 1 XCIX'. p. 42, pl. 12.

in number 13:13-50; all fixed in deep and distinct sockets, separated by interspaces which admit of the close interlocking of the upper and lower teeth when the mouth is closed; the longest and largest teeth are at the middle of the series, and they gradually decrease in size as they approach the ends, especially the posterior one.

In the common Dolphin the number of teeth amount to 190, arranged in equal numbers above and below, and there is a pair of teeth in the premaxillaries which are toothless in the other Cetacea. They have slender, sharp, conical, slightly incurved crowns, and diminish in size to the two extremes of the dental series; the acute apices are longer preserved than in the foregoing species.

The Gangetic Dolphin (Platanista gangetica) differs from the rest of the Delphinide scarcely less in the form of its teeth than in that of the jaws. Both the upper and lower maxillary bones are much elongated and compressed; the symphysis of the lower jaw is coextensive with the long dental series, and the teeth rise so close to it that those of one side touch the others by their bases, except at the posterior part of the jaw. The lateral series of teeth are similarly approximated in the upper jaw at the median line of union, which line is compelled, by the alternate position of the teeth, to take a wavy course. There are thirty teeth on each side of the upper jaw, and thirty-two on each side of the lower jaw. In the young animal they are all slender, compressed, straight, and sharp-pointed, the anterior being longer than the posterior ones, and recurved. Contrary to the rule in ordinary Dolphins, the anterior teeth retain their prehensile structure, while the posterior ones soon have their summits worn down to their broad bases: in the progress of their growth the implanted base is elongated antero-posteriorly, its outer surface augmented by longitudinal folds analogous to those in the teeth of the Sauroid fishes. Sometimes the posterior tooth of Platanista has the base divided into two short fangs, the sole example of such a structure which I have met with in the existing carnivorous Cetacea. In the Dolphins of the South American rivers (Inia) the inner side of the tooth expands into a crushing tubercle.

The primitive seat of the development of the tooth-matrix is maintained longer in the Cetacea than in other Mammalia; a greater portion of the tooth is also developed before the matrix sinks into, or is surrounded by, a bony alveolus; and, with the exception of the rudimental tusks in the Narwhal, is at no period

entirely closed in a bony cell, in which respect the Cetacea offer an interesting analogy to true fishes. $220. Teeth of Diphyodonts. A. Sirenia.-Two marks of inferiority in the dental system of the carnivorous Cetacea, which they have in common with many of the order Bruta, viz. a general uniformity of shape in the whole series of teeth, and no succession and displacement by a second or permanent set, disappear when we commence the examination of the dentition of those apodal pachyderms which were called by Cuvier the Herbivorous Cetacea.

In the Dugong (Halicore), for example, we find incisors distinguished by their configuration as well as position from the molars, and the incisive tusk is deciduous, displaced vertically, and succeeded by a permanent tusk; both these characters are shown in fig. 160, vol. ii. p. 281. Of the incisors of the Dugong, only the superior ones project from the gum in the male sex, and neither upper nor lower ones are visible in the female. The superior incisors, ib. i, are two in number in both sexes. In the male they are moderately long, subtriedral, of the same diameter from the base to near the apex, which is obliquely bevelled off to a sharp edge, like the scalpriform teeth of the Rodentia. Only the extremity of this tusk projects from the jaw, at least seven-eighths of its extent being lodged in the socket, the parietes of which are entire. In the female Dugong the growth of the permanent incisive tusks of the upper jaw is arrested before they cut the gum, and they remain throughout life concealed in the premaxillary bones; the tusk in this sex is solid, is about an inch shorter and less bent than that of the male; it is also irregularly cylindrical, longitudinally indented, and it gradually diminishes to an obtuse rugged point; the base is suddenly expanded, bent obliquely outwards, and presents a shallow excavation. The deciduous incisors of the upper jaw, i, d, are much smaller than the permanent tusks of the female, and are loosely inserted by one extremity in conical sockets immediately anterior to those of the permanent tusks, adhering by their opposite ends to their tegumentary gum, which presents no outward indication of their presence. Not more than twenty-four molar teeth are developed in the Australian Dugong (Halicore Australis), or more than twenty molar teeth in the Malayan Dugong, viz., in the latter, five on each side of both upper and lower jaws, ib. 1-5, but these are never simultaneously in use, the first being shed before the last has cut the gum.

The molar teeth of the Dugong consist of a large body of dentine, a small central part of osteo-dentine, and a thick external investment of cement, c, fig. 242, vol. i. p. 365. In the female

Dugong the whole of the smaller extremity of the tusk is surrounded by a thin coat of true enamel, which is covered by a thinner stratum of cement. In the male's tusk the enamel, though it may originally have capped the extremity, as in the female's, yet, in the body of the tusk, it is laid only upon the anterior convex, and on the lateral surfaces, but not upon the posterior concave side of the tusk, which is thickly coated with cement. This side, accordingly, is worn away obliquely when the tusk comes into use, whilst the enamel maintains a sharp chisellike edge upon the anterior part of the protruded end of the tusk.

The presence of abortive teeth concealed in the sockets of the deflected part of the lower jaw of the Dugong, fig. 160, a, i, d (vol. ii.), offers an analogy with the rudimental dentition of the upper jaw in the Cachalot, and of both jaws in the foetal Whales. The arrested growth and concealment of the upper tusks in the female Dugong, and the persistent pulp-cavity and projection of the corresponding tusks in the male, are equally interesting repetitions of the phenomena manifested on a larger scale in the dental system of the Narwhal. The simple implantation of the molar teeth and their composition are paralleled in the teeth of the Cachalot; their difference of form, and the more complex shape of the hindmost tooth, ib. b, are repetitions of characters which were present in the dentition of the extinct Zeuglodon. coexistence of incisive tusks with molar teeth, and the successive displacement of the smaller and more simple anterior ones by the advance of larger and more complex grinders into the field of attrition, already seem to sketch out peculiarities of dentition which become established and attain their maximum in the Proboscidian family (Elephants and Mastodons) of the Ungulates.

The

The molars of the American Manatee are thirty-eight in number, ten on each side of the upper jaw, and nine, at least, on each side of the lower jaw; but they are never simultaneously in place and use. The first in both jaws is small and simple. Beyond the second, the crowns in the upper jaw are square, and support two transverse ridges with tri-tuberculate summits, having also an anterior and posterior basal ridge; each tooth is implanted by three diverging roots, one on the inner and two on the outer side: they increase in size very gradually, from the foremost to the last. The crowns of the four or five anterior molars of the lower jaw resemble those above, but the rest have a large posterior tubercle; they are all implanted by two fangs which enlarge as they descend, and bifurcate at the extre mity; the crowns are of moderate height, and project only a few