Premolars sentence example
Specialized species like Mastodon americanus have completely lost the rudimentary premolars.
Premolars compressed, pointed; and the molars with quadrate tuberculated crowns.
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.
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.Advertisement
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.Advertisement
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.
Other modifications are the loss of the upper incisors; the development of the canines into projecting tusks; and the loss of the anterior premolars.
As regards classification, the first group is that of the Pecora, or Cotylophora, in which the cheek-teeth are selenodont, but there are no upper incisors or canine-like premolars, Pecora.
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.Advertisement
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..
Teeth variable in number, owing to the suppression in some forms of an upper incisor and one or more premolars.
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.Advertisement
When there are more than three cheek-teeth, those which precede the last three have succeeded milk-teeth, and are premolars.
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.
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.
The second section, Castoroidea, of the present group includes only the family Castoridae, represented by the beavers, which are large aquatic rodents characterized by their massive skulls, devoid of post-orbital processes, with the angle of the lower jaw rounded, the molars rootless or semi-rooted, with re-entering enamel-folds, and one pair of premolars above and below.
There is a single pair of premolars in each jaw.Advertisement
In general habits and appearance these animals recall large jerboas, from which group they are, however, distinguished by the four pairs of rooted cheek-teeth, the premolars being as large as the molars, and the latter having one outer and one inner enamel-fold.
In the dormice, forming the section Myoxidea, with the single family Gliridae (or Myoxidae), a single pair of premolars may or may not be present; the molars are short-crowned and rooted, with transverse From de Winton.
None of the members of the typical sub-family extend into India, where the group is represented by Platacanthomys, typifying the sub-family Platacanthomyinae, characterized by the absence of premolars; the other being the Chinese Typhlomys.
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.
In the typical jerboas, Jaculus (or Dipus), ranging from North Africa to Persia, Russia and Central Asia, there are only three hind toes, the incisors are grooved, and the premolars are generally wanting.Advertisement
Euchoreutes, with one Yarkand species, has premolars, enormous ears and a long nose.
The Turkestan Platycercomys (or Pygeretmus) has a lancet-shaped tail and no premolars; while Cardiocranus of the Nan-shan district of Central Asia has a similar type of tail, but short ears and a peculiarly triangular skull.
At least one pair of premolars is present in each jaw; and these teeth and the molars typically have one outer and one inc_e.
The most remarkable members of the family are the sand-rats of Somaliland and Shoa, forming the genera Heterocephalus and Fornarina, in which the premolars may be reduced to two pairs.
The palate is so much contracted in front that the premolars of opposite sides touch by their antero-internal edges.Advertisement
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 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 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.
There is generally little gap between the canines and the premolars.Advertisement
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 skull, which has a longer face than in Titanotherium, lacks horn-cores, while all the upper premolars are simpler than the molars, and the full series of 44 teeth was present.
Another of these titanotheroid forms is Diplacodon, from the Upper or Uinta Eocene; an animal the size of a rhinoceros, with the last two upper premolars molar-like.
The crowns of the molars belong to the crescentic or selenodont type, and are tall-crowned or hypsodont; but one or more of the anterior premolars is usually detached from the series, and of simple pointed form.
The teeth of the cheek-series which are in contact with each other consist of two small premolars (the first almost rudimentary) and three broad molars, constructed generally like those of Camelus.Advertisement
Homacodon was an animal of the size of a rabbit, with five toes (of which only five were functional to each foot) and 44 teeth, of which the molars are tuberculated (bunodont), with six columns on those of the upper jaw; the premolars being of a cutting type.
The dental formula, when completely developed, is incisors i, canines o, premolars 31 molars - on each side, giving a total of 34 teeth.
The first two premolars are compressed, with cutting longitudinal edges, the anterior one is deciduous, being lost about the time the second one replaces the milk-molar, so that three premolars are never found in place and use in the same individual.
In Macropus giganteus and its immediate allies, the premolars and sometimes the first molar are shed, so that in old examples only the two posterior molars and the incisors are found in place.
The molars are usually not longer (from before backwards) than the anterior premolars, and less compressed than in the next section.Advertisement
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.
In the members of the typical genus Potorous (formerly known as Hypsiprymnus) the head is long and slender, with the auditory bullae somewhat swollen; while the ridges on the first two premolars are few and perpendicular, and there are large vacuities on the palate.
There are eight pairs of teeth, the first four of which are simpler than the rest, and may perhaps therefore be regarded as premolars.
The remaining four are the " premolars."
This system has been objected to as artificial, and in many cases not descriptive, the distinction between premolars and canine especially being sometimes not obvious; but the terms are now in such general use, and also so convenient, that it is not likely they will be superseded.
With regard to the lower teeth the difficulties are greater, owing to the absence of any suture corresponding to that which defines the incisors above; but since the number of the teeth is the same, since the corresponding teeth are preceded by milk-teeth, and since in the large majority of cases it is the fourth tooth of the series which is modified in the same way as the canine (or fourth tooth) of the upper jaw, it is reasonable to adopt the same divisions as with the upper series, and to call the first three, which are implanted in the part of the mandible opposite to the premaxilla, the incisors, the next the canine, the next four the premolars, and the last three the molars.
When, as is the case among nearly all existing mammals with the exception of the members of the genera Sus (pigs), Gymnura (ratshrew), Talpa (moles) and Myogale (desmans) the number of teeth is reduced below the typical forty-four, it appears to be an almost universal rule that if one of the incisors is missing it is the second, or middle one, while the premolars commence to disappear from the front end of the series and the molars from the hinder end.
The teeth which precede the premolars of the permanent series are called either milk-molar or milk-premolar.
When there is a marked difference between the premolars and molars of the permanent dentition, the first milk-molar resembles a premolar, while the last has the characters of the posterior molar.
The most remarkable feature about the marsupial dentition is that, at most, only a single pair of teeth is replaced in each jaw; this pair, on the assumption that there are four premolars, representing the third of that series.
In the specialized forms the premolars tend to become more or less completely like the molars; and, contrary to what obtains among the Carnivora, the whole series of cheek-teeth (with the occasional exception of the first) is very strongly developed.
The anterior premolars are quite rudimentary, sometimes not developed at all, and generally fall by the time the animal attains maturity, so that there are but six functional cheek teeth, - three that have predecessors in the milk-dentition, and hence are considered as premolars, and three molars, but otherwise, except the first and last of the series, not distinguishable in form or structure.
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.
In all the dentition is of the hypsodont type, with the hollows of the cheek-teeth filled by cement, the premolars molariform, and the first small and generally deciduous.
The general characteristics of this progenitor of the horses are those given above as distinctive of the group. The cheek-teeth are, however, much simpler than those of Anchitherium; the transverse crests of the upper molars not being fully connected with the outer wall, while the premolars in the upper jaw are triangular, and thus unlike the molars.
Assessment The maxilla was too narrow for a natural tongue resting position, having had four premolars removed at age 12.
One hundred forty two first maxillary premolars (142 teeth) were examined.
Hooks most commonly form on the front edge of the upper first premolars and the back edge of the lower last molars.
The examined teeth were 218 permanent premolars from archeological sites in the south - west of Poland.
Our study indicates that 15.5% of the first maxillary premolars had a single root.
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.
As in the following genera, there are two pairs of premolars, of which the first in this case is small and rounded, while the two series of cheek-teeth are nearly parallel (see SousLIK).
When fully developed, the number of cheek-teeth is, however, seven; and it is probable that, as in placentals, the first four of these are premolars and the remaining three molars, although it was long held that these numbers should P. ? ??
Severe overcrowding may necessitate the extraction of permanent teeth, usually the premolars, to create space prior to using braces to move teeth.
They consist of wires and springs that are held in place by small brackets glued to the outside surfaces of the incisors and sometimes the premolars.
Molars-The teeth behind the primary canines or the permanent premolars, with large crowns and broad chewing surfaces for grinding food.
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.
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.
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.
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.
The maximum number of teeth is 36, there being typically two pairs of incisors and three of premolars in each jaw.
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.
The incisors are chisel-shaped, and the canines tend to become isolated, so as in the more specialized forms to occupy a more or less midway position in a longer or shorter gap between the incisors and premolars.
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.
The dentition includes one pair of premolars above and below, and rooted or rootless molars with but few enamel folds.
In both genera there is only a single pair of premolars in each jaw, but in the smaller Myoscalops there are usually three pairs of these teeth.