In all cases a more or less full series of teeth is developed, these being differentiated into incisors, canines, premolars and molars, when all are present; but only a single pair of teeth in each jaw has deciduous predecessors.
Premolars with compressed crowns, increasing in size from before backwards.
First two premolars with compressed and sharp-pointed crowns, and slightly developed anterior and posterior accessory basal cusps.
Premolars compressed, pointed; and the molars with quadrate tuberculated crowns.
The first upper incisor is much larger than the others; canine and first two premolars rudimentary.
In the lower jaw there are also one or two small and early deciduous premolars; third premolars of both jaws formed on the same type as that of the rat-kangaroos, but relatively much larger; molars rudimentary, tubercular.
Inclined forwards; canines, upper small or moderate, conical I or o and sharp-pointed; lower absent or rudimentary; premolars variable; molars 3, or 2 i with four obtuse tubercles, sometimes.
These ancient opossums have been separated generically from Didelphys (in its widest sense) on account of certain differences in the relative sizes of the lower premolars, but as nearly the whole of the species have been formed .on lower jaws, of which some hundreds have been found, it is impossible to judge how far these differences are correlated with other dental or osteological characters.
It may be added that the division of these teeth into premolars and molars in figs.
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.
Unlike the early horses, the later premolars are as complex as the molars; and although there is a well-marked gap between the canine and the premolars, there is only a very short one between the former and the incisors.
The cheek-teeth (premolars and molars) form a A B C FIG.
Continuous series, with massive, quadrate, transversely ridged or complex crowns--the posterior premolars usually resembling the molars in structure.
In the Equidae the premolars are generally or.
The upper cheek-teeth are short-crowned and without cement, and show distinct traces of the primitive tubercles; the two outer columns form a more or less complete external wall, connected with the inner ones by a pair of nearly straight transverse crests; and the premolars are originally simpler than the molars.
Hinder premolars either simple or molar-like.
In North America the earliest representative of the group is Systemodon of the Lower Eocene, in which all the upper premolars are quite simple; while the molars are of a type which would readily develop into that of the modern tapirs, both outer columns being conical and of equal size.
6 a Oligocene of both hemispheres appears Protapirus, which ranges well into the Miocene, and is essentially a tapir, having lost the third lobe of the last lower molar, and being in process of acquiring molar-like upper premolars, although none of these teeth have two complete inner columns.
Finally, Tapirus itself, in which the last three upper premolars, makes its appearance in the Upper Miocene, and continues till the present day.
The other upper premolars and molars all formed on the same plan and of nearly the same size, with four roots and quadrate crowns, rather wider transversely than from before backwards, each having four columns, connected by a pair of transverse ridges, anterior and posterior.
In America the family is represented by Heptodon, of the Middle Eocene, which differs from the early members of the tapir-stock in having a long gap between the lower canine and first premolar; the dentition is complete, and the upper premolars are simple.
The next stage is Helaletes, also of Middle Eocene age, in which the first lower premolar has disappeared, and the last two upper premolars have become molar-like.
Finally, in the Oligocene Colodon the last three upper premolars are like the molars, and the first pair of lower incisors is lost.
In Europe the group is represented by the long-known and typical genus Lophiodon with three premolars in each jaw, of which the upper are simpler than the molars.
In the Amynodontidae, represented by the North American Middle Eocene Amynodon and Metamynodon, the premolars may be either 4 or g, making the total number of teeth either 44 or 40.
As regards the dentition of the existing species, the cheek-series consists of the four premolars and three molars above and below, all in contact and closely resembling each other, except the first, which is much smaller than the rest and often deciduous; the others gradually increasing in size up to the penultimate.
Sibircum of the Siberian Pleistocene, in which the premolars were reduced to while front-teeth were probably wanting, and the cheek teeth developed tall crowns, without roots, but with cement in the valleys, and the enamel of the central parts curiously crimped.
The cheek-teeth are selenodont, and one pair of upper incisors is retained, while some of the anterior premolars assume a canine-like shape, and are separated from the rest of the cheekseries.
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.
Xiphodon and Dichodon represent another type with cutting premolars and selenodont molars; while Caenotherium and Plesiomeryx form yet another branch, with resemblances to the ruminants.
Finally, we have in the Pliocene of India the genus Tetraconodon, remarkable for the enormous size attained by the bluntly conical premolars; as the molars are purely bunodont, this genus seems to be a late and specialized survivor of a primitive type..
Titanotherium, of the Oligocene of the Dakotas and neighbouring districts, was a huge beast, with the hinder upper premolars similar in character to the molars, a pair of horn-cores, arising from the maxilla, overhanging the nose-cavity, four front and three hind toes, only twenty dorso-lumbar vertebrae, and an almost continuous and unbroken series of teeth, in which the canines are short; the dental formula being i.
The earlier forms had the full series of 44 teeth, with the premolars simpler than the molars; but in the later types the canines and some of the incisors disappear, and at least the hinder premolars become molar-like.
The teeth form a continuous even series, the small canines being crowded between the incisors and premolars; the crowns of the cheek-series are tall (hypsodont), with a distinctive pattern of their own.
The palate is so much contracted in front that the premolars of opposite sides touch by their antero-internal edges.
In Europe these form the genus Ischyrornys and the family Ischyromyidae, and have premolars i, and all the cheek-teeth low-crowned, with simple cusps or ridges.
A more advanced phase is represented in the European Lower Oligocene by the Pseudosciuridae, with the genera Pseudosciurus, Sciuroides, Trechomys, Theridomys, &c., in which part of the masseter passes through the broad infra-orbital canal, and the premolars are; the molars being low-crowned, many-rooted and either cusped or ridged.
The premolars and molars may be rooted or rootless, with tuberculated or laminated crowns, and are arranged in an unbroken series.
The palate is narrow from before backwards, this being especially the case in the hares, where it is reduced to a mere bridge between the premolars; in others, as in the rodent-moles (Bathyerginae), it is extremely narrow transversely, its width being less than that of one of the molar teeth.
Sewellels are medium-sized terrestrial rodents, with no postorbital process to the skull, which is depressed in form, and rootless cheek-teeth, among which the premolars number I, the first in the upper jaw being very small.
5), and very generally two upper premolars, making a total of five pairs of upper cheek-teeth, which have crowns of medium height.
The Nannosciurinae, or second sub-family of Sciuridae, are represented only by the pigmy squirrels (Nannosciurus), characterized by their very short-crowned molars (which approximate to those of dormice in structure) and small premolars, of which the first upper pair is often deciduous, while the upper molars have only three oblique ridges.