Teeth are fixed as a general rule in all Vertebrata, and the only known exceptions are those presented by certain species of fishes, e.g. the Sharks, Lophioids, Gonio donts. In the higher Vertebrata the move ments of the teeth depend on those of the jaw-bones to which they are affixed, but appear to be independent in the ratio of the size of the tooth to the bone to which it is attached. Thus the extent of rotatory movement to which the large perforated poison fangs of the rattle-snake are subject depends upon the rotation of the small maxillary bone. So likewise the seemingly individual movements of divarication and approximation observable in the large lower incisors of the Bathyergus and Macropus#, are due entirely to the yielding nature of the symphysis uniting the two rami of the lower jaw in which those incisors are deeply and firmly implanted. It is no more a property of the teeth themselves than is that alternate removal of the lower teeth from, and bringing of them in contact with, the upper teeth of the mouth, which one sees or feels in the act of mastication.
True teeth implanted in sockets are con fined, in the Mammalian class, to the maxil lary, premaxillary, and mandibular, or lower maxillary bones, and form a single row in each. They may project only from the pre maxillary bones, as in the Narwhal, or only from the lower maxillary bone, as in Ziphius; or be apparent only in the lower maxillary bone, as in the Cachalot ; or be limited to the superior and inferior maxillaries, and not pre sent in the premaxillaries, as in the true Pecora, and most Bruta of Linnaeus ; in general, teeth are situated in all the bones above mentioned. In Man, where the pre maxillaries early coalesce with the maxillary bones, where the jaws are very short and the crowns of the teeth are of equal length, there is no interspace or "diastema " in the dental series of either jaw, and the teeth derive some additional fixity by their close apposi tion and mutual pressure. No inferior Mam mal now presents this character ; but its im portance, as associated with the peculiar attributes of the human organisation, has been somewhat diminished by the discovery of a like contiguous arrangement of the teeth in the jaws of a few extinct quadrupeds : e. g. Anoplotherium, Nesodon, and Diehodon.* The teeth of the Mammalia usually consist of hard unvascular dentine, defended at the crown by an investment of enamel, and every where surrounded by a coat of cement. The coronal cement is of extreme tenuity in Man, Quadrumana, and terrestrial Carnivora ; it is thicker in the Herbivora, especially in the complex grinders of the Elephant ; and is thickest in the teeth of the Sloths, Mega therioids, Dugong, Walrus, and Cachalot. Vertical folds of enamel and cement penetrate the crown of the tooth in the Ruminants, and in most Rodents and Pachyderms, charac terising by their various forms the genera of the last two orders ; but these folds never converge from equidistant points of the cir cumference of the crown towards its centre.
The teeth of the quadrupeds of the order Bruta (Edentata, Cuv.) have no true enamel; this is absent likewise in the molars of the Dugong and the Cachalot+ The tusks of the Narwhal, Walrus, Dinotherium, Masto don, and Elephant consist of modified den tine, which, in the last two great proboscidian animals, is properly called "ivory," t and is covered by cement.
In the subjoined magnified view of a section of the molar of a Megatherium, t is the hard dentine, v the vaso-dentine, and c the cement (jig. 574).
The teeth in the Mammalia, as in the fore going classes, are formed by superaddition of the hardening salts to pre-existing moulds of animal pulp or membrane, organised so as to insure the arrangement of the earthy particles according to that pattern which characterises each constituent texture of the tooth.
The complexity of the primordial basis, or matrix, corresponds, therefore, with that of the fully-formed tooth, and is least remark able in those conical teeth which consist only of the same form and diameter throughout, except in the immature animal, when it widens of dentine and cement. The primary pulp, which first appears as a papilla rising from the ree surface of the alveolar gum, is the part of the matrix which by its calcification consti tutes the dentine ; it sinks into a cell and becomes surrounded by a closed capsule in every matnmiferous species, at an early stage of the formation of the tooth ; and, as the cement is the result of the ossification of the capsule, every tooth must be covered by a layer of that substance. In those teeth which possess enamel, the mould or pulp of that constituent is developed from the capsule covering the coronal part of the dentinal pulp. In the simple teeth the secondary or enamel pulp covers the crown like a cap ; in the complex teeth it sends processes into depres sions of the crown, which vary in depth, breadth, direction, and number in the nu merous groups of the herbivorous and om nivorous quadrupeds. The dentinal pulp, thus penetrated, offers corresponding com plications of form ; and as the capsule follows the enamel pulp in all its folds and processes, the external cavities or interspaces of the dentine become occupied by enamel and ce ment ; the cement, like the capsule which formed it, being the outermost substance, and the enamel being interposed between it and the dentine. The dental matrix presents the most extensive interdigitation of the dentinal and enamel pulps in the Capybara and Ele phant. The processes of formation and cal cification of the several constituents of Mam malian teeth will be found described in the Introduction to my " Odontography "* and in the article TOOTH.