The complexity of the structure of the crown of the molar teeth, and the quantity of enamel and cement interblended- with tbe dentine, are greatest-in the rootless molars of the strictly herbivorous Rodents. The crowns of the rooted molars of the omnivorous rats and mice are almost as simple as the tuber culate molars of the bear, or of the human subject, which they appear to typify. They are at first tuberculate ; when the summits of the tubercles are worn off, the inequality of the grinding surface is for a time maintained by the deeper transverse folds of enamel, the margins of which are separated by alternate valleys of dentine and cement ; but these folds, sinking only to a slight depth, are in time obliterated, and the grinding surface is reduced to a smooth field of dentine, with a simple border of enamel. A similar change in the grinding surface, consequent on age and use, is shown in the molars of the souslik, or ground squirrel; as also in those of the gerbille, and is common to all that possess roots. It will be seen that these folds have a general tendency to a trans verse direction across the crown of the tooth. Baron Cuvier has pointed out the concomitant modification of the shape of the joint of the lower jaw, which almost restricts it to horizontal movements to and fro, in the direction of the axis of the head, during the act of mastication. When the folds of enamel dip in vertically from the summit to a greater or less depth into the substance of the crown of the tooth, as in those molars which have roots, the con figuration of the grinding surface varies with the degree of abrasion, of which examples have the Guinea-pig, the capybara, and the Pata gonian cavy.
The whole exterior of the molar teeth of the Rodentia is covered by a cement, and the external interspaces of the enamel folds are filled with the same substance. In the chin chilidm and the capybara, where the folds of enamel extend quite across the body of the tooth, and insulate as many plates of dentine, these detached portions are held together by the cement ; such folds of enamel are usually parallel, as in the large posterior lower molar of the capybara, which, in shape and structure, offers a very close and interesting resemblance to the molars of the Asiatic elephant.
The partial folds and islands of enamel in the molars of the porcupine and agouti, typify the structure of the teeth of the rhi noceros ; the opposite lateral inflections of enamel in the molars of the gerbille and Cape mole-rat represent the structure of the molars of the hippopotamus ; the double crescentic folds in the jerboa sketch out, as it were, the characteristic structure of the molars of the Anoplothere and Ruminantia.
Although, as has been shown, the molar teeth in many Rodents are rootless and of un limited growth, as in the Edentata, in none is enamel absent ; or vascular dentine, as the chief constituent of the tooth. present. These essential differences characterise the molars of those Rodents, which by use have their grinding surface reduced to a simple depres sion hounded by a raised circular margin, as in the great Cape mole; that margin being formed by true enamel, but in the sloths by hard dentine.
It is peculiar to some of the Rodents with rootless molars to have the sockets of these long curved teeth open at both extremities, so that, in the dry skull, the base of the tooth protrudes as well as the grinding surface ; the matrix in such instances adheres to the peri osteum, which covered the portion of bone absorbed from the bottom of the alveolus.
The jumping hare (He!ways eapensis), when full grown, offers a good example of this cu riou s structii re.
The molars are not numerous in any Ro dents ; the hare and rabbit (Lepus) have 6-6 • i.e. six molars on each side of the upper jaw, and five on each side of the lower jaw : 5-5 the pika (Lagomys), has ; the squirrels hhave • t e families of the dormice, the ' porcupines, the spring rats (Echingida),the octodonts, chinchillas, and cavies, have 4-4 4 lars ; in the great family of rats (Ilittridce), the normal number of molars is 3-3 • but the Australian water rat (Hydro3-3 ' nip) has but 2-2 molars, making with the 2-2 incisors twelve teeth, which is the smallest number in the Rodent order ; the greatest number of teeth in the present order is twenty eight, which is exemplified in the hare and rabbit ; but thirty-six teeth are developed in these species, six molars and two incisors being deciduous.
In all the Rodents, in which the number of molars exceeds three in a series, the addi tional ones are anterior to these, and are pre molars, i. e. they have each displaced a deci duous predecessor in the vertical direction, and are what Cuvier calls dents de rempktce ment. This it is which constitutes the essential distinction between the dentition of the marsupial and the placental Rodent ; the latter, like the placental Carnivore Ru minantia, and ordinary Pachydermata having never more than three true molars. Thus the Rodents, which have the molar formula of 4-4 shed the first tooth in each series and this is succeeded by a permanent pre-molar, which comes into place later than the true molars ; later, at least, than the first and second, even when the deciduous molar is shed before birth, as was observed by Cu vier in the Guinea-pig. In the hare and rabbit the three anterior teeth in the upper jaw, and the two anterior ones in the lower jaw, succeed and displace, in like manner, de ciduous predecessors, and come into place after the first and second true molars are in use, and contemporaneously with the last molar.
It does not appear that the scalpriform incisors are preceded by milk teeth, or, like the pre-molars of the Guinea-pig, by uterine teeth ; but the second incisor was observed by Cuvier to be so preceded in the genus Lepus, and he has figured the jaw of a young rabbit, before that deciduous tooth was shed, when six incisors are present in the upper jaw. This condition is interesting, both as a transi tory manifestation of the normal number of incisive teeth in the 3Iammalia series, and as it elucidates the disputed nature of the great anterior scalpriform teeth. Geoffroy St. Hilaire contended that the scalpriform teeth of the Rodents were canines, because those of the upper javr extended their fang backwards into the maxillary bone, which lodged part of their hollow base and matrix. But the scalpriform teeth are confined exclusively to the inter maxillary bones at the beginning of their formation; and the smaller incisors, which are developed behind them in our anomalous native Rodents, the hare and rabbit, retain their usual relations with the interrnaxillaries, and, a fortiori, prove the tooth which projects anterior to them to be also an incisor.