In some cases the molar-formula is 3.
The molar teeth are six in number on each side, increasing in size from before backwards, and, as in the elephants, with a horizontal succession, the anterior teeth being lost before the full development of the posterior ones, which gradually move forward, taking the place of those that are destroyed by wear.
The humerus lacks a foramen at the lower end; and the molar teeth, as explained and illustrated in the article Vole, consist of two longitudinal rows of triangular alternating vertical prisms, and may be either rootless or rooted.
13, B); in the upper teeth the outer cusps and in the lower the inner ones are the higher, and when worn the crown surfaces show oblique dentineareas; in shape the third molar is like the second, but it is smaller.
The average number of plates of the six successive molar teeth may be expressed by the "ridge-formula" 4, 8, 12, 12, 16, 24.
Cypriotes; but since their molar teeth are essentially miniatures of those of the African elephant, it has been suggested by later observers that these animals are nothing more than dwarf races of the latter.
- Upper Molar of Mastodon arvernensis, viewed from below.
Hinder premolars either simple or molar-like.
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.
Lophiodochoerus apparently represents this stage in the European Lower Eocene; Isectolophus, of the American Middle Eocene, represents a distinct advance, the last upper premolar becoming molar-like, while a second species from the Upper Eocene is still more advanced; the third lobe is, however, retained in the last lower molar.
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.
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.
The lower cheekteeth have, unlike those of the Tapiroidea, crescentic ridges, which have not the loops at their extremities characteristic of the advanced Hippoidea; the last lower molar has no third lobe.
With the exception of the first lower premolar, the dentit i on is complete; the incisors being normal, but the canine rudimentary, and the last upper molar distinctly triangular.
The incisors tend to become latera l, the canines are enlarged, and the last upper molar is sub-quadrangular.
They differ from the other American monkeys in having one pair less of molar teeth in each jaw.
- Section of Upper Molar Teeth of Megatherium.
Unlike the Bovinae, there are frequently glands in the feet; and the upper molar teeth differ from those of that group in their narrower crowns, which lack a distinct inner column.
It is chiefly by the characters of the molar teeth that the various extinct modifications of the elephant type are distinguished.
- Grinding surface of Upper Molar Tooth of the7Mammoth (Elephas primigenius).
In later memoirs Reynolds followed up this subject by proceeding to establish definitions of the velocity and the momentum and the energy at an element of volume of the molecular medium, with the precision necessary in order that the dynamical equations of the medium in bulk, based in the usual manner on these quantities alone, without directly considering thermal stresses, shall be strictly valid - a discussion in which the relation of ordinary molar mechanics to the more complete molecular theory is involved.
UNGULATA, the name of an order of placental mammals in which the terminal joints of the toes are usually encased in solid hoofs or covered with broad hoof-like nails, while the molar (and not unfrequently some or all of the premolar) teeth have broad tuberculated crowns adapted for crushing vegetable substances.
The upper premolar and molar teeth are not alike, the former being single and the latter two-lobed; and the last lower molar of both first and second dentition is almost invariably threelobed.
As regards the teeth, we have the passage of a simply tubercular, or bunodont ((30vv6s, a hillock) type of molar into one in which the four main tubercles, or columns, have assumed a crescentic form, whence this type is termed selenodont (v€X vn, the new moon).
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.
A large trade is carried on, by way of the Orebro canal and lakes Hjelmar and Molar, with Stockholm.
In the genus Tejus the teeth of the adult become molar-like; and in Dracaena they are transformed into large, oval crushers, indicating strictly herbivorous habits, while most members of the family live upon animal food.
- TEN Stages In The Evolution Of The Second Upper Molar Tooth Of The Right Side, Arranged According To Geological Level.
The crown of the third molar is nearly as long as those of the first and second together, having, in addition to the four principal lobes, a large posterior heel, composed of clustered conical cusps, and supported by additional roots.
The third or last molar tooth of both jaws is of great size, and presents a structure at first sight unlike that of any other mammal, being composed of numerous (22-25) parallel cylinders or columns, each with pulp-cavity, dentine and enamel-covering, and packed together with cement.
Examination will, however, show that a modification similar to that which has transformed the comparatively simple molar tooth of the mastodon into the extremely complex grinder of the Indian elephant has served to change the tooth of the common pig into that of Phacochoerus.
In the loss of the last upper molar, the Liu-Kiu rabbit approximates to the picas, as does the tailless rabbit in the abortion of its caudal appendage.
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.
6) remarkably heavy, the molar teeth differing from those of Spermophilus and Arctomys by having three instead of two transverse grooves on their crowns.
The front root of the zygomatic arch is nearly vertical, and placed so far back that it is above the second molar, while the orbit - a unique feature among rodents - is almost completely surrounded by bone.
Avellanarius, the common dormouse, distinguished by the cylindrical bushy tail, and thickened glandular walls of the cardiac extremity of the oesophagus; thirdly, Eliomys, containing several species, with tufted and doubly vaned tails, simple stomachs and smaller molar teeth, having concave crowns and faintly marked enamel-folds; and lastly, the African Graphiurus, represented by several species, with short cylindrical tails ending in a pencil of hairs, and very small molars almost without trace of enamel-folds.
They are small rat-like rodents, with one pair of upper premolars, which are mere pins, as is the last molar, and the two pairs of limbs of normal length, with the metatarsals separate; the infra-orbital opening in the skull being triangular and widest below, while the incisive foramina in the palate are elongated.
The large-eared African Otomys and the allied Oreomys (Oreinomys), often made the type of a distinct sub-family, may be included in this section; as well as the small African tree-mice, Dendromys, allied to which is Deomys, peculiar in the circumstance that only the first molar has three rows of cusps, the other two having only a couple of such rows, as in cricetines.
The dentition is peculiar on account of the great size and complexity of the last upper molar, which is composed of about twelve plates, and exceeds in length the three teeth in front.
The teeth of the molar series gradually increase in size and complexity from first to last, and are arranged in contiguous series, except that the first lower premolar is separated by an interval from the second.
Crown of the last lower molar commonly bilobed.
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.