muscles, to act with greater force, are called holders, tearers, laniaries, or more commonly canines, (ib. c,) from being well developed in the Dog and other Carnivora.

It is peculiar to the class Mammalia to have teeth implanted in sockets by two or more fangs; but this can only happen to teeth of limited growth, and generally characterizes the molars and premolars: perpetually growing teeth require the base to be kept simple and widely excavated for the persistent pulp. In no mammiferous animal does anchylosis of the tooth with the jaw constitute a normal mode of attachment. Each tooth has its peculiar socket, to which it firmly adheres by the close co-adaptation of their opposed surfaces, and by the firm adhesion of the alveolar periosteum to the organized cement which invests the fang or fangs of the tooth.

True teeth implanted in sockets are confined, in the Mammalian class, to the maxillary, premaxillary, and mandibular or lower maxillary bones, and form a single row in each. They may project only from the premaxillary bones, as in the Narwhal; or only from the lower maxillary bone, as in Ziphius; or be limited to the superior and inferior maxillaries and not present in the premaxillaries, as in the true Ruminantia and most Bruta (Sloths, Armadillos, Orycteropes). In most Mammals teeth are situated in all the bones above mentioned.

The teeth of the Mammalia usually consist of hard unvascular dentine, defended at the crown by an investment of enamel, and everywhere surrounded by a coat of cement.

The coronal cement is of extreme tenuity in Man, Quadrumana and the terrestrial Carnivora; it is thicker in the Herbivora, especially in the complex grinders of the Elephant.

Vertical folds of enamel and cement penetrate the crown of the tooth in the ruminating and many other Ungulata, and in most Rodents, characterizing by their various forms the genera of those orders.

No Mammal has more than two sets of teeth. In some

species the tooth-matrix does not develope the germ of a second tooth, destined to succeed the one into which the matrix has been converted; such a tooth, therefore, when completed and worn down, is not replaced. The Sperm Whales, Dolphins, and Porpoises are limited to this simple provision of teeth. In the Armadillos and Sloths, the want of generative power, as it may be called, in the matrix is compensated by the persistence of the matrix, and by the uninterrupted growth of the teeth.

In most other Mammalia, the matrix of the first-developed tooth gives origin to the germ of a second tooth, which sometimes displaces the first, sometimes takes its place by the side of the tooth from which it has originated..

All those teeth which are displaced by their progeny are called 'temporary,' deciduous, or milk-teeth, (figs. 2 and 3, d, 1...4); the mode and direction in which they are displaced and succeeded, viz. from above downwards in the upper, from below upwards in the lower, jaw, in both jaws vertically—are the same as in the Crocodile; but the process is never repeated more than once in any mammalian animal. A considerable proportion of the dental series is thus changed; the second or 'permanent' teeth having a size and form as suitable to the jaws of the adult, as the 'temporary' teeth were adapted to those of the young animal.

Those permanent teeth, which assume places not previously occupied by deciduous ones, are always the most posterior in their position, and generally the most complex in their form. The term 'molar' or 'true molar' is restricted to these teeth (fig. 2 and 3, m). The teeth between them and the canines are called 'premolars,' (ib. p); they push out the milk-teeth, (ib. d,) and are usually of smaller size and simpler form than the true molars.

Thus the class Mammalia, in regard to the times of formation and the succession of the teeth, may be divided into two groups, viz. Monophyodonts1 or those that generate a

1 μóvos, once; púw, I generate; ¿doùs, tooth.

single set of teeth, and the Diphyodonts' or those that generate two sets of teeth. But this dental character is not so associated with other organic characters as to indicate natural or equivalent sub-classes.

In the Mammalian orders with two sets of teeth, these organs acquire individual characters, receive special denominations, and can be determinated from species to species. This differentiation of the teeth is significative of the high grade of organization of the animals manifesting it.

Originally, indeed, the names 'incisors,' 'canines,' and 'molars,' were given to the teeth, in Man and certain Mammals, as in Reptiles and Fishes, in reference merely to the shape and offices indicated by those names; but they are now used as arbitrary signs, in a more fixed and determinate sense. In some Carnivora, e. g., the front teeth have broad tuberculate summits adapted for nipping and bruising, while the principal back-teeth are shaped for cutting and work upon each other like the blades of scissors. The front-teeth in the Elephant project from the upper jaw, in the form, size and direction of long pointed horns. Indeed, shape and size are the least constant of dental characters in the Mammalia; and the homologous teeth are determined, like other parts, by their relative position, by their connexions, and by their development.

Those teeth which are implanted in the premaxillary bones, and in the corresponding part of the lower jaw, are called "incisors' (fig. 2, i), whatever be their shape or size. The tooth in the maxillary bone, which is situated at or near to the suture with the premaxillary, is the 'canine,' as is also that tooth in the lower jaw (ib. c), which, in opposing it, passes in front of the upper one's crown when the mouth is closed. The other teeth of the first set are the 'deciduous molars' (d. 1—3); the teeth which displace and succeed them vertically are the 'premolars' (p. 1—3); the more posterior

1 dis, twice; púw and ỏdous. See Philosophical Transactions, 1850, p. 493.

C

teeth, which are not displaced by vertical successors, are the 'molars' properly so called (m. 1-4).

Lower Jaw of a young Opossum (Didelphys).

I have been led, chiefly by the state of the dentition in most of the early forms of both carnivorous and herbivorous Mammalia, which flourished during the eocene tertiary periods, to regard 3 incisors, 1 canine, and 7 succeeding teeth, on each side of both jaws, as the type formula of diphyodont dentition.

Three of the seven teeth may be 'premolars' (fig. 2, p. 1-3), and four may be true molars' (ib. m. 1-4); or there may be four premolars (fig. 3, p. 1-4), and three true molars (ib. m. 1-3). This difference forms a character of an

ordinal group in the mammalian class1. The essential nature of the distinction is as follows: true molars (ib. m.) are a backward continuation of the first series of teeth (ib. d.); they are developed in the same primary groove of the foetal gum; they are 'permanent' because they are not pushed out by the successional teeth (ib. p.), called 'dents de remplacement' by Cuvier. Seven teeth developed in the primary groove is,

1 Outlines of a Classification of the Mammalia, Trans. Zool. Soc. Vol. II. p. 330 (1839).

therefore, the typical number of first teeth, beyond the canines. If, as in Didelphys (fig. 2), the anterior three develope toothgerms which come to perfection in a 'secondary groove,' there are then 3 deciduous teeth, 3 premolars, and 4 true molars: if, as in Sus, fig. 3, the anterior four of the 'primary' teeth develope tooth-germs, which grow in a secondary groove, there are then 4 deciduous teeth, 4 premolars, and 3 true molars. The first true molar of the marsupial (fig. 2, m. 1, d. 4), is thus seen to be the homologue of the last milk-molar of the placental (fig. 3, d. 4).

The Hog, the Mole, the Gymnure and the Opossum, are among the few existing quadrupeds which retain the typical number and kinds of teeth. In a young Hog of ten months (fig. 3), the first premolar, p. 1, and the first molar, m. 1, are in place and use together with the three deciduous molars, d. 2, d. 3, and d. 4; the second molar, m. 2, has just begun to cut the gum; p. 2, p. 3, and p. 4, together with m. 3, are more or less incomplete, and will be found concealed in their closed alveoli1.

The last deciduous molar, d.4, has the same relative superiority of size to d. 3 and d. 2, which m. 3 bears to m. 2 and m. 1; and the crowns of p. 3 and p. 4 are of a more simple form than those of the milk-teeth, which they are destined to succeed. When the milk-teeth are shed, and the permanent ones are all in place, their kinds are indicated, in the genus Sus, by the following formula :

which signifies that there are on each side of both upper and lower jaws 3 incisors, 1 canine, 4 premolars, and 3 molars, making in all 44 teeth, each tooth being distinguished by its appropriate symbol, viz. p. 1 to p.4, m.1 to m.3. This number of teeth is never surpassed in the placental diphyodont series.

1 I recommend this easily acquired 'subject' to the young zoologist for a demonstration of the most instructive peculiarities of the mammalian dentition. He will see that the premolars must displace deciduous molars in order to rise into place the molars have no such relations.