These three are therefore reckoned as milk-molars, and their successors as premolars, while the last three correspond to the true molars of other mammals.
It also shows that the anterior molars of elephants do not correspond to the premolars of other ungulates, but to the milk-molars, the early loss of which in consequence of the peculiar process of horizontal forward-moving (From Owen.) FIG.
In elephants there are only two, the last milk-molar and the first true molar (or the third and fourth of the whole series), which are alike in the number of ridges; whereas in mastodons there are three such teeth, the last milk-molar and the first and second molars (or the third, fourth and fifth of the whole series).
In elephants the number of ridges on the intermediate molars always exceeds five, but in mastodons it is nearly always three or four, and the tooth in front has usually one fewer and that behind one more, so that the ridgeformula (i.e.
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.
Attained its full stature, and is not shed and replaced by its successor until after all the other teeth, including the molars, are in place and use.
On the other hand, there are those who believe that the functional dentition (other than the replacing premolar and the molars) correspond to the milk-dentition of placentals, and that the rudimentary tooth-germs represent a "prelacteal" dentition.
Molars in general characters resembling those of Sarcophilus, but of more simple form, the cusps being less distinct and not so sharply pointed.
Molars with numerous sharp-pointed cusps.
In the lower jaw the molars more compressed, with longer cusps; the last not notably smaller than the others.
Total 46 - and in having the teeth generally developed upon an insectivorous rather than a carnivorous pattern, the upper middle incisors being larger and inclined forward, the canines relatively smaller, and the molars with broad crowns, armed with prickly tubercles.
S8 °19 or 99 The teeth are all small and (except the four posterior inferior molars) separated from each other by an interval.
Premolars compressed, pointed; and the molars with quadrate tuberculated crowns.
The three pairs of molars in each jaw are, like the last premolar, quadritubercular oblong teeth.
Canine very small; a considerable interval between it and the first premolar, which is as long from before backwards but not so broad as the molars, and has a cutting edge, with a smaller parallel inner ridge.
- 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.
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.
The broad molars are either bluntly tuberculated or transversely ridged; the outer side of the hind part of the lower jaw has a deep pocket; and the hind-limbs are generally very long, with the structure of the foot similar to that of the bandicoots.
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.
The cheek-teeth are short-crowned, generally with no cement, the upper molars having a W-shaped outer wall, from which proceed two oblique transverse crests, while the lower ones carry two crescents.
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 full series of forty-four teeth was developed; and the upper molars were short-crowned, or brachyodont, with six low cones, two internal, two intermediate.
There are but three pairs of incisor teeth in each jaw, and the upper molars are tricuspid.
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.
There is usually a short gap between the canine and first premolar; the upper molars are short-crowned and transitional between the bunodont (tubercular) and selenodont (crescentic) types, with two outer concave tubercles and two inner conical ones; while the lower molars are crescentic, with three lobes in the last of the series..
In the upper molars the two outer columns or tubercles of the primitive tubercular molar coalesce to form an outer wall, from which proceed two crescentic transverse crests, the connexion between the crests and the wall being slight or imperfect, and the crests themselves sometimes tubercular.
Each of the lower molars carries two crescentic ridges.
In the earlier short-crowned forms these teeth are unlike the molars, and the first of the series is separated by a gap from the second.
Outer columns of upper molars similar, the hinder ones not flattened; ridges of lower molars oblique or directly transverse, a third ridge to the last molar in the earlier forms. The Lophiodontidae, which date from the Eocene, come very close to Hyracotherium in the horse-line; and it is solely on the authority of American palaeontologists that the division of these early forms into equoids and tapiroids is attempted.
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.
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.
Nearly related is the extinct family Lophiodontidae (inclusive of the American Helaletidae), in which both the upper and lower first premolar may be absent, while the upper molars present a more rhinoceros-like form, owing to the lateral compression and consequent lengthening of the outer columns, of which the hinder is bent somewhat inwards and is more or less concave externally, thus forming a more complete outer wall.
Finally, in the Oligocene Colodon the last three upper premolars are like the molars, and the first pair of lower incisors is lost.
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.
The upper molars present a characteristic pattern of crown, having a much-developed flat or more or less sinuous outer wall, and two transverse ridges running obliquely inwards and backwards from it, terminating internally in conical eminences or columns, and enclosing a deep valley between.
As stated above, the lower molars have the crown formed by a pair of crescents; the last having no third lobe.
- Grinding Surface of moderately worn Right Upper Second Molars of Rhinoceros.
The tall upper molars have inner columns.
The molars, as in other elephants, are six in number on each side above and below, succeeding each other from before backwards.
The full typical series of 44 teeth was developed in each, but whereas in the Periptychidae the upper molars were bunodont and tritubercular, in the Pantolambdidae they have assumed a selenodont structure.
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 upper molars, which may be either selenodont or buno-selenodont, carry five cusps each, instead of the four characteristic of all the preceding groups; and they are all very low-crowned, so as to expose the whole of the valleys between the cusps.
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.
The molars are partially selenodont in the typical genus Anthracotherium, with five cusps, or columns, on the crowns of those of the upper jaw, which are nearly square.
In Ancodon (Hyopotamus) the cusps on the molars are taller, so that the dentition is more decidedly selenodont; the distribution of this genus includes not only Europe, Asia and North Africa, but also Egypt where it occurs in Upper Eocene beds in company with the European genus Rhagatherium, which is nearer Anthracotherium.
On the other hand, in Merycopotamus, of the Lower Pliocene of India and Burma, the upper molars have lost the fifth intermediate cusp of Ancodon; and thus, although highly selenodont, might be easily modified, by a kind of retrograde development, into the trefoil-columned molars of Hippopotamus.
They are heavily built ruminants, with horns of nearly equal size in both sexes, short tapering tails, large hoofs, narrow goat-like upper molars, and usually small face-glands.