The upper incisor teeth are generally marked by grooves.
The first upper incisor is much larger than the others; canine and first two premolars rudimentary.
In the more typical Lemuridae there are two pairs of upper incisor teeth, separated by a gap in the middle line; the premolars may be either two or three, but the molars, as in the lower jaw, are always three on each side.
The third incisor in both upper and lower jaws is large, developed before the others, with much the size, form and direction of the canine.
Its affinity with the giraffes is, however, clearly revealed by the structure of the skull and teeth, more especially the bilobed crown to the incisor-like lower canine teeth.
The first in the upper jaw is strong, curved and cutting, the other two generally somewhat smaller; the single lower functional incisor large, more or less.
Lower incisors diminishing in size from the first to the third; the canine, which is in contact with the third incisor, large and conical, working against (and behind) the canine-like third upper incisor.
Beavers are sociable animals, living in streams, where, so as to render the water of sufficient depth, they build dams of mud and of the stems and boughs of trees felled by their powerful incisor teeth.
The upper jaw is apparently destitute of incisor and canine teeth, but possesses five molars on each side, with a corresponding number in the jaw beneath.
M', m 2, and m 3, The three true and The three incisor teeth.
Both upper and lower incisors are regularly curved, the upper ones: slightly more so than the lower; and, their growth being continuous, should anything prevent the normal wear by which their length is regulated - as by the loss of one of them, or by displacement owing to a broken jaw or other cause - the unopposed incisor may gradually curve upon itself until a complete circle or more has, been formed, the tooth sometimes passing through some part of the animal's head.
As a general rule, the depression caused by the infolding of the surface of the incisor (the " mark ") is obliterated in the first or central incisor at six years, in the second at seven years, and in the third at eight years.
Finally, we have the family Rhinocerotidae, which includes the existing representatives of the group. In this family the dentition has undergone considerable reduction, and may be represented inclusive of all the variations, by the formula i a or a m a The first upper incisor, whenpresent, has an 430r2; PP antero-posteriorly elongated crown, but the second is small; when fully developed, the lower canine is a large forwardly directed tusk-like tooth with sharp cutting-edges, and biting against the first upper incisor.
RODENTIA, or Glires, an order of placental mammals characterized by the peculiar form and structure of their front or incisor teeth, which are reduced to a single functional chisellike pair in each jaw, specially adapted for gnawing, and growing throughout the entire life of their owners.
- In the skull there is a sagittal crest; the tympanic bulla is filled with cancellous tissue; the condyle of the lower jaw is rounded; and the premaxillae, or anterior bones of the upper jaw, have the full number of incisor teeth in the young state, the outermost of these being persistent through life as an isolated tooth.
In Protolabis of the Middle Miocene, while no cannon-bone is formed, the first and second pairs of incisor teeth are retained, and the limbs and feet are short and disproportionately small.
Here the metacarpals and metatarsals have partially united to form cannonbones, the skull has assumed the elongated form characteristic of modern camels, with the loss of the first and second pairs of upper incisors, and the development of gaps in front of and behind each of the next three teeth, that is to say, the third incisor, the canine and the first cheek-tooth.
The most characteristic dental feature is, however, the assumption of the form and function of a canine by the first lower premolar; the lower canine being incisor-like.
In the rat-kangaroos, or kangaroo-rats, as they are called in Australia, constituting the sub-family Potoroinae, the first upper incisor is narrow, curved, and much exceeds the others in length; the upper canines are persistent, flattened, blunt and slightly curved, and the first two premolars of both jaws have large, simple, compressed crowns, with a nearly straight or slightly concave free cutting-edge, and both outer and inner surfaces usually marked by a series of parallel, vertical grooves and ridges.
These three teeth, which are implanted in the premaxilla, form a distinct group, to which the name of " incisor " is applied.
The latter expand in front, and are curved downwards to form the semicircular alveolar border which supports the large incisor teeth.
The palate is narrow in the interval between the incisor and molar teeth, in which are situated the large anterior palatine foramina.
The horizontal ramus, long, straight, and compressed, gradually narrows towards the symphysis, where it expands laterally to form with the ankylosed opposite ramus the wide, semicircular, shallow alveolar border for the incisor teeth.
3 - Longitudinal and Transverse Section dark from its 3 - g dark colour, and con of Upper Incisor of Horse.
The crown it becomes obliterated as the latter wears away, and then the tooth assumes the character of that of an ordinary incisor, consisting only of a core of dentine, surrounded by the external enamel layer.
Between two and a half and three years the first incisor appears.
At three years the second and third premolars, and the third molar have appeared, at from three and a half to four years the second incisor, at four to four and a half years the canine, and, finally, at five years, the third incisor, completing the permanent dentition.
The outermost incisor is short, the bones of the middle part of the leg are separate, and there are at least three toes to each foot.
The tusks, or upper incisor teeth, which were probably smaller in the female, in the adult males attained the length of from 9 to io ft.
The primitive Artiodactyla thus probably had the typical number (44) of incisor, canine and molar teeth, brachyodont molars, conical odontoid process, four distinct toes on each foot, with metacarpal, metatarsal and all the tarsal bones distinct, and no frontal appendages.
The cheek-teeth are selenodont, as in the two preceding groups; there are no upper incisors, but there are long, narrow and pointed upper canines, which attain a large size in the males; the lower canines are incisor-like, as in the Pecora, and there are no caniniform premolars in either jaw.
In the latter there is only one pair of incisor teeth in the upper jaw, in which the enamel is confined to the front surface.
This animal, whose remains occur in the Lower Pliocene of both Attica and Samos, was about the size of a donkey, and possessed three pairs of upper incisor teeth, of which the innermost were large and trihedral, recalling those of the existing genus.
Brontops seem scarcely separated from the type genus; but the name Brontotherium is applied to species with two pairs of incisor teeth in both jaws.
Pecora, or true ruminants as they may be conveniently called, have complex stomachs and chew the cud; they have no upper incisor teeth; and the lower canines are approximated to the outer incisors in such a manner that the three incisors and the one canine of the two sides collectively form a continuous semicircle of four pairs of nearly similar teeth.
In the upper jaw there is a compressed, sharp-pointed, tusk-like incisor near the hind edge of the premaxilla, followed in the male at least by a moderate-sized, pointed, curved canine in the anterior part of the maxilla.
The two points thus meeting, the bill is 1 This peculiarity is found as an accidental malformation in the crows (Corvidae) and other groups; it is comparable to the monstrosities seen in rabbits and other members of the order Glires, in which the incisor teeth grow to an inordinate length.
The three incisors of the upper jaw are arranged in a continuous arched series, and have crowns with broad cutting edges; the first or middle incisor is often larger than the others.
The basal line of the cranium from the lower border of the foramen magnum to the incisor border of the palate is nearly straight.
The first teeth which appear are the first and second milk-molars (about five days), then the central incisor (from seven to ten days); this is followed by the second incisor (at one month), then the third molar, and finally the third incisor.
As a sub-order, the Paucituberculata are characterized by the presence of four pairs of upper and three of lower incisor teeth; the enlargement and forward inclination of the first pair of lower incisors, and the presence of four or five sharp cusps on the cheek-teeth, coupled with the absence of "syndactylism" in the hind limbs.
It may be added that a few traces of mammals have been obtained from the English Wealden, among which an incisor tooth foreshadows the rodent type.
There are but three pairs of incisor teeth in each jaw, and the upper molars are tricuspid.
In this group the incisors and canines are very variable in number and form; the lower canine being separated by only a short gap from the outer incisor (when present), but by a long one from the first premolar, which is in contact with the second.
Teeth variable in number, owing to the suppression in some forms of an upper incisor and one or more premolars.
- Vertical and Longitudinal Section through the Skull of the Beaver (Castor fiber), showing the brain-cavity, the greatly developed plates of bone in the nose-cavity, the mode of implantation of the ever-growing chisel-edged incisor, and the curved rootless cheek-teeth.
- Skeleton of Right incisor very large and chisel-like, Hind-Foot of Koala (Phas- molars with prominent transverse colarctus cinereus), showing ridges, as in Macropus, but without stout opposable hallux, folthe longitudinal connecting ridge.