developed in the dog and other Carnivora, although they are given, likewise, to many vegetable feeders for defence or combat ; e. g., Musk-deer. Molar teeth, which are adapted for mastication, have either tuberculate, or ridged, or flat summits, and usually are either surrounded by a ridge of enamel, or are traversed by similar ridges arranged in various patterns. Certain molars in the Dugong, the Mylodon, and the Zeuglodon, are so deeply indented laterally by opposite longitudinal grooves, as to appear, when abraded, to be composed of two cylindrical teeth cemented together, and the transverse section of the crown is bilobed. The teeth of the Glyptodon were fluted by two analogous grooves on each side, fig. 214. The large molars of the Capybara and Elephant have the crown cleft into a numerous series of compressed transverse plates, cemented together side by side. The modifications of the crown of the molar teeth are those that are most intimately related to the kind of food of the animal possessing them. Thus, in the purely carnivorous mammals, the principal molars are simple, trenchant, and play upon each other like scissor-blades. In the mixed feeding species, the working surface of the molars becomes broader and tuberculated; in the insectivorous species it is bristled with sharp points; and in the purely herbivorous kinds, the flat grinding surface of the teeth is complicated by folds and ridges of the enamel entering the substance of the tooth, the most complex forms being presented by the Elephants.

211

E

§ 219. Teeth of Monophyodonts. A. Monotremata. - The substances serving for teeth in the Ornithorhynchus are of a horny texture, consisting of close-set, vertical hollow tubes, resembling the outer compact tissue of baleen or whalebone.' They are eight in number, four in the upper, and as many in the under jaw. The anterior tooth of the upper jaw is extended from behind forward, but is low, very narrow, and four-sided.

The

corresponding tooth in the lower jaw, fig. 211, b, is rather narrower, and retains longer its trenchant edge. At a distance from the anterior tooth, equal to its own length, is situated the horny molar, ib. c, which consists of a flattened plate of an oblong subquadrate figure. The corresponding tooth in the lower jaw is

Mandible and teeth, Ornithorhynchus.

somewhat narrower, but of simple form. Each division or tubercle of the molar is separately developed, and they become confluent in the course of growth. According to the analysis of Lassaigne, 99.5 parts of the dental tissue of the Ornithorhynchus have the composition of horn; this is hardened by 0-3 parts of phosphate of lime.

The notice of the dental apparatus of the Monotremes ought to include mention of the two short and thick conical processes, fig. 212, g, g, which project from the forepart of the raised intermolar portion of the tongue, in the Ornithorhynchus; and like the more

212

numerous spines on the corresponding part of the tongue of the Echidna, represent, in these loworganised mammals, the lingual teeth of fishes.

B. Bruta. The teeth of the Orycterope, or Cape Ant-eater, are of a simple form, but peculiar structure; their common number in the mature animal is 3:26, and they all belong to the molar series. The first and smallest is soon lost. The proportions of the persistent teeth, the depth of their sockets, and their structure, as viewed in longitudinal section with the naked

Tongue, lingual teeth, and larynx of the Ornithorhynchus.

Section of lower jaw and teeth of the Orycteropus. Nat. size

eye, are shown in fig. 213. The teeth are continued, solid, and of the same dimensions, to the bottom of the socket, and terminate in a truncate and undivided base. If each be viewed as an aggregate of teeth, as partially shown in fig. 247, vol. i., p. 396, it will be found that the component denticle has its base excavated by a conical pulp-cavity, as in other animals, and which is persistent, as in the rest of the order Bruta. The wide inferior apertures of these pulp-cavities constitute the pores observable on the base of the compound tooth of the Orycterope, and give to that part a close resemblance to the section of a cane. The canals to which these pores lead are the centres of radiation of the dentinal

tubes; such denticles are cemented together laterally, ib. c, slightly decreasing in diameter, and occasionally bifurcating as they approach the grinding surface of the tooth. The substance of the entire tooth thus resembles the teeth of the Myliobates and Chimeroids among fishes, rather than any in the Mammalian class, in which it offers a transitional step from the horny dental substitutes, above described, to the true teeth.

The teeth of the Orycteropus, when rightly understood, offer, however, no anomaly in their mode of formation. Each denticle is developed according to the same laws, and by as simple a matrix, as those larger teeth in other mammals which consist only of dentine and cement. The dentine is formed by calcification of the pulp, the cement by ossification of the capsule; both pulp and capsule continue to be reproduced at the bottom of the alveolus, pari passu with the attrition of the exposed crown; and the mode and time of growth being alike in each denticle, the whole compound tooth is maintained thoughout the life of the animal. The augmentation in the size of the whole tooth, during the growth of the jaw, is effected by the development of new denticles, and a slight increase of size in the old ones, at the base of the growing tooth, which, in the progress of attrition and growth, becomes its grinding surface.

The teeth of the Armadillo-tribe are harder than those of other species of Bruta, the unvascular dentine being present in greatest proportion, and forming the main body of the tooth; it includes a small central axis of vascular dentine, and is surrounded by an extremely thin coating of cement. The numerous teeth in Priodon are of very small size and simple form, and are all referable to the molar series. They vary in number from twenty-four to twenty-six in each upper jaw, and from twenty-two to twenty-four on each side of the lower jaw, amounting to from ninety-four to one hundred in total number. The Armadillos of the sub-genus Euphractus, Wagler, are distinguished by having the anterior tooth, which is shaped like the succeeding molar, implanted in the premaxillary bone. The two anterior teeth of the lower jaw being in advance of the premaxillary tooth, are, with it, arbitrarily held to be incisors.

214

extinct Armadillo

d

Some species of the extinct loricate genus, Glyptodon, surpassed the Rhinoceros in size, Crown of tooth of great and the dentition was more complicated, and (Glyptodon clavipes). more adapted to a vegetable diet, than that of the small existing Armadillos. The osteo-dentine, fig. 214, o, occupied a larger

proportion of the centre of the tooth, and being harder than the dentine, d, or cement, c, rose upon the grinding surface, in the form of a ridge extending along the middle of the long axis of that surface, and in three shorter ridges at right angles to the preceding, at the middle of each of the three rhomboidal divisions of the tooth.

Of the leaf-eating species of the order Bruta, very few, and these the most diminutive of the tribe, now exist. The following are the characters of their dentition, both recent and extinct :--Teeth implanted in the maxillary and mandibular bones, few in number, not exceeding :; composed of a large central axis of vaso-dentine, with a thin investment of hard dentine, and a thick outer coating of cement: to these add the dental characters common to the order Bruta, viz., uninterrupted growth, and concomitant implantation by a simple, deeply-excavated base.

215

In the two-toed sloth (Cholapus didactylus, Illig.) the teeth, fig. 215, offer a greater inequality of size than has yet been observed in any other genus of Bruta; the first of each series, 1, in both jaws, which in the rest of the order is the smallest, here so much exceeds the others as, with its peculiar form, to have received the name of a canine. This tooth is separated by a marked interval from the other teeth, 2-5, especially in the upper jaw, so that 1-1 above play upon the anterior part of those below, contrary to the relative position and mutual action of the true canine teeth in the Quadrumana and Carnivora.

The teeth of the Megatherium, the most gigantic of the extinct quadrupeds of the Sloth tribe, are five in number on each side of the upper jaw, fig. 216, and four on each side of the lower jaw. They are deeply implanted with narrow intervals: each is excavated by an unusually extensive pulp-cavity, ib. p, from the apex of which a fissure is continued to the middle depression of the grinding surface of the tooth. The central axis of vaso-dentine, v, is surrounded by a thin layer of hard or unvascular dentine, d, and this is coated by the cement, c, which is of great thickness on the anterior and posterior surfaces, but is thin where it covers the outer and inner sides of the tooth. The vaso-dentine, v, fig. 238,

vol. i. p. 361, is traversed throughout by medullary canals, measuring of an inch in diameter, continued from the pulpcavity, and anastomosing in pairs by a loop, the convexity of which is turned towards the origin of the tubes of the hard dentine, t.

Section of upper jaw and teeth of the Megatherium. One-third nat. size.

The cement, ib. c, is characterised by the size, number, and regularity of the vascular canals which traverse it in a direction slightly inclined from the transverse axis toward the crown of the tooth, running parallel to each other, and anastomose in loops, the convexity of which is directed toward the hard dentine.

The tooth of the Megatherium offers an unequivocal example of a course of nutriment from the dentine to the cement, and reciprocally. All the constituents of the blood freely circulated through the vascular dentine and the cement, and the vessels of each substance, intercommunicated by a few canals, continued across the hard or unvascular dentine. The minuter tubes, which pervade every part of the tooth, characterising by their difference of length and course the three constituent substances, form one continuous and freely intercommunicating system of strengthening and reparative vessels, by which the plasma of the blood was distributed throughout the entire tooth, for its nutrition and maintenance in a healthy state.

The grinding surface of the close-set molars of the Megatherium differs on account of the greater thickness of the cement on their