The molar teeth of the Dugong resemble those of the order Bruta in the total absence of enamel, and of any constriction defining the crown from the fangs. In the Malayan species, only five molars (fig. 575, 1, 2, 3, 4, 5.) are developed on each side of both jaws : in the Australian Dugong six are developed ; i. e. the Halicore indicus is characterised by 5-5 the molar formula ni. — 5-5=20, whilst the Halicore australis has ni. But in both species, the number is progressively re duced, by the shedding of the anterior and smaller molars, to ni. The struc ture of these molar teeth is illustrated in fig. 546. a, their form in fig. 546. ; the last molar, when it comes into use, presents a bibbed form of grinding surface, as is shown at b, fig. 575.
Owing to there being but one set of molars in the Dugong, those teeth cannot be divided into true and false molars, any more than in the Sloths or Armadillos. In the true Di phyodonts, in which each kind of teeth have deciduous predecessors, those grinders which succeed the deciduous ones vertically, and displace them, are called "premolars," or " false molars," and those that come into place behind these, without pushing out vertically any predecessors, are the " molars proper," or " true molars." In this article, as in my " the two sorts of grinders are calleZrespectively " premolars " and " molars." In the Marsupial order the normal number of molars is four in each dental series, i.e. m. in the placental Diphyodonts their normal number is three, ; the normal number of pre molars in the Marsupialia is but in the Placentalia, it is 4-1 — : in both the numerical 4 11 character of the canines is one, i. e. that 3— of the incisors three, i. e. As regards 3---3 the latter teeth, however, the number of exceptions in the Marsupialia is considerable, and the incisors are sometimes in excess ; whilst in the placental Diphyodonts, the in-. cisors never exceed the typical number, but frequently depart from it by suppression or arrest of development.
In fishes and reptiles, certain teeth might be called " incisive," " laniary," or " molar" teeth, in reference to the special adaptation of their form for cutting, tearing, or bruising; but such terms, in the cold-blooded classes, imply no thing more than those modifications of form ; they are not significative of constant and well defined groups of teeth, and could not become the names of definite parts or organs determin able and traceable from one species to another. In the Mammalian orders, with two sets of teeth, these organs acquire fixed individual characters, receive special denominations, and can be determined from species to species. This individualisation of the teeth is emi nently significative of the high grade of organ isation of the animals manifesting it ; espe cially when we consider the great proportion of mineral substance which enters into the composition of those parts ; in the number and nature of which the principle of vegetative repetition, and the power of the general polarising forces, have been most controlled in the Mammalia.
Originally, indeed, the name " incisors," " laniaries ' or " canines," and " molars" were given to the teeth, in Man and certain mammals, as in reptiles, in reference merely to the shape and offices so indicated ; but they are now used as arbitrary signs, in a more fixed and determinate sense. In some Carnivora, e.g., the front teeth have broad tuberculate summits, adapted for nipping and bruising, while the principal back teeth are shaped for cutting, and work upon each other like the blades of scissors. The front teeth in the elephant project from the upper jaw, in the form, size, and direction of long pointed horns. In short, shape and size are the least constant of dental characters in the Mam malia; and the homologous teeth are deter mined, like other parts, by their relative position, by their connections, and by their development.
Those teeth which are implanted in the premaxillary bones, and in the corresponding part of the lower jaw, are called " incisors," whatever be their shape or size. The tooth in the maxillary bone, which is situated at, or near to, the suture with the premaxillary, is the " canine," as is also that tooth in the lower jaw which, in opposing it, passes in front of its crown when the mouth is closed. The other teeth of the first set are the "de ciduous molars ; " the teeth which displace and succeed them vertically are the " pre molars ;" the more posterior teeth, which are not displaced by vertical successors, are the " molars " properly so called.
When the premolars and the molars are below their typical number, the absent teeth are missing from the fore-part of the pre molar series and from the back part of the molar series. The most constant teeth are the fourth premolar and the first true molar ; and, these being known by their order and mode of development, the homologies of the remaining molars and premolars are deter mined by counting the molars from before backwards, e.g. "two," "three;" and the premolars from behind forwards, e. g. " four," " three," "two," "one." The incisors are counted from the median line, commonly the foremost part of both upper and lower jaws, outwards and backwards. The first in cisor of the right side is the homotype, trans versely, of the contiguous incisor of the left side in the same jaw, and, vertically, of its opposing tooth in the opposite jaw ; and so with regard to the canines, premolars, and molars ; just as the right arm is the homo type of the left arm in its own segment, and also of the right leg of a succeeding segment. It suffices, therefore, to reckon and name the teeth of one side of either jaw in a species with the typical number and kinds of teeth ; e.g. the first, second, and third incisors,— the first, second, third, and fourth premolars, —the first, second, and third molars ; and of one side of both jaws in any case.