The incisors* are always regularly curved, the upper ones describing a larger segment of a smaller circle, the lower ones a smaller segment of a larger circle; these are the longest incisors, and usually have their alveoli ex tended below or on the inner side of those of the molars to tbe back part of the lower jaw. Like the molars of the Me gatherium, and other teeth of unlimited growth, the implanted part of the long and large incisors retains the form and size of the exposed part or crown to the widely open base, which contains a long, conical, persistent dentinal pulp, and is surrounded by the cap sule in a progressive state of ossification as it approaches the crown, an enamel pulp being attached to the inner side of that part of the capsule which covers the convex surface of the curved incisor. The matrix is here no ticed in connection with the tooth, because it is always found in full development and activity- to the time of the Rodent's death. The calcification of the dentinal pnlp, the de position of the earthy salts in the cells of the enamel pulp, and the ossification'of the capsule proceed contemporaneously ; fresh materials being added to the base of the vas cular matrix as its several constituents are progressively converted into the dental tissues in the more advanced part of the socket. The tooth thence projecting consists of a body of compact dentine, sometimes with a few short medullary canals continued into it from the persistent pulp cavity, with a plate of enamel laid upon its anterior surface, and a general investment of cement, which is very thin upon the enamel, but less thin in some Rodents, upon the posterior and lateral parts of the incisor. The substance of the incisor diminishes in hardness from the front to the back part of the tooth ; the enamel consisting of two layers, of which the anterior and ex ternal is denser than the posterior layer, and the posterior half of the dentine being by a modified number and arrangement of the caleigerous tubes less dense than the anterior half.
The abrasion resulting from the reciprocal action of the upper and lower incisors pro.
duces accordingly an oblique surface, sloping from a sharp anterior margin formed by the dense enamel, like that which slopes from the sharp edge formed by the plate of hard steel laid upon the back of a chisel ; whence the name dentes scalprarii given to the incisors of the Rodentia.
The varieties to which these incisors are subject in the different Rodents are limited to their proportional size, and to the colour and sculpturing of the anterior surface. Thus in the Guinea-pig, jerboa, and squirrel the breadth of the incisors is not half so great as that of the molars, whilst in the coypa they are as broad, and in the Cape mole rats (Bathyergus and Orycteromys) broader than the molars.
In the coypa, beaver, agouti, and some other Rodents, the enamelled surface of the incisors is of a bright orange or reddish brown colour. In some genera of Rodents, as orycteromys, otomys, meriones, gerbilla, hydrochwrus, lepus, and lagomys, the anterior surface of the upper incisors is indented by a deep longitudinal groove. This character seems not to influence the food or habits of the species ; it is often present in one genus and absent in another of the same natural family; in mostRodents the anterior enamelled surface of the scalpriform teeth is smooth and uniform.
The molar teeth are always few in number, obliquely implanted and obliquely abraded, the lateral series converging anteriorly in both jaws ; but they present a striking con trast to the incisors in the range of their varieties, which are so numerous that they typify almost all the modifications of form and structure vvhich are met with in the molar teeth of the omnivorous and her bivorous genera of other orders of Mammalia.
In some Rodents the molar teeth are root less, like those of the wombat, the toxodon, and elasmothere; some have short roots tardily developed, like the molars of the horse and elephant ; and some soon acquire roots of the ordinary proportional length.
The Rodents which have rootless molars comprise the families of the hares, chin chillas, Chili rats, and cavies ; most of the voles, the houtias (Capromys), and the Cape jerboa (Helamys).
The genera which have molars, with short or incomplete roots, developed late, are Castor (beaver), Hyshix (porcupine), Calogenys (spotted cavy), Dasyproeta (agouti), Spalax (blind rat), Illyopotamus (coypa), Euryotis, Accomys, and Aptodontia.
The families of the squirrels, dormice, rats, and jerboas have rooted molars, The differences in the mode of implantation of the molar teeth relate to differences of diet. The Rodents, which subsist on mixed food, and which betray a tendency to carnivorous habits, as the true rats, or which subsist on the softer and more nutritious vegetable sub stance, as the oily kernels of nuts, suffer less rapid abrasion of the molar teeth ; a minor depth of the crown is therefore needed to perform the office of mastication during the brief period of existence allotted to these active little mammals ; and as the economy of nature is manifested in the srnallest par ticulars as well as in her grandest operations, no more dental substance is developed after the crown is formed than is requisite for the firm implantation of the tooth in the jaw.
Rodents that exclusively subsist on vege table substance, especially the coarser and less nutritious kinds, as herbage, foliage, the bark and wood of trees, wear away more rapidly the grinding surface of the molar teeth ; the crowns are therefore larger, and their growth continues by 'a reproduction of the formative matrix at their base, in propor tion as its calcified constituents, forming the exposed working part of the tooth, are worn away. So long as this reproductive force is active the molar tooth is implanted, like the incisor, by a long undivided continuation of the crown ; when the force begins to be ex hausted the matrix is simplified by the sup pression of the enamel organ, and the dentinal pulp continues to be reproduced only at certain points of the base of the crown, which by their elongation constitute the fangs. The beaver and other Rodents in the second cate gory of the order, according to the implanta tion of the molar teeth, exemplify the above condition ; but in the capybara, dolichotis, and other Rodents with rootless molars, the reproduction of the molar, like that of the in cisor teeth, appears to continue throughout the animal's existence. The rootless and per petually growing molars are always more or already been cited ; but in the rootless molars, where the folds of enamel extend inwards from the entire length of the sides of the tooth, the characteristic configuration of the grinding surface is maintained without variation, as in less curved ; they derive from this form the same advantage as the incisors, in the relief of the delicate tissues of the active vascular matrix from the effects of the pressure which would otherwise have been transmitted more directly from the grinding surface.