Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/376

Rh 354 MAMMALIA [DENTAL SYSTEM, situation, relations, and development of a tooth may be required before its nature can be determined, and in some cases the evidence at our disposal is scarcely sufficient for the purpose. The milk dentition is expressed by a similar formula, d for deciduous being commonly added before the letter expressive of the nature of the tooth. As the three molars and (almost invariably x ) the first premolar of the permanent series have no predecessors, the typical milk dentition would be expressed as follows di f, dc y, dm = 28. The teeth which precede the premolars of the permanent series are all called molars in the milk dentition, although, as a general rule, in form and function they represent in a condensed form the whole premolar and molar series of the adult. When there is a marked difference between the premolars and molars of the permanent dentition, the first milk molar resembles a premolar, and the last has the characters of the posterior true molar. Excep- The dentition of all the animals of the orders Primates, tiona Carnivora, Insectivora, Chir&ptera, and Ungulata can from clearly be derived from the above-described generalized ized type. The same may be said of the llodents, and even type. the Proboscidea, though with greater modification, at least in the existing members of the order. It is also apparent in certain extinct Cetaceans, as Zeuglodon and Squalodon, but it is difficult to find any traces of it in existing Cetacea, Sirenia, or any of the so-called Edentata. All the Marsupials, different as they are in their general structure and mode of life, and variously modified as is their dentition, present in this system of organs some deep-lying common characters which show their unity of origin. The generalized type to which their dentition can be reduced presents considerable resemblance to that of the placental mammals, yet differing in details. It is markedly heterodont, and susceptible of division into incisors, canines, premolars, and molars upon the same principles. The whole number is, however, not limited to forty-four. The incisors may be as numerous as five on each side, and are almost always different in number in the upper and the lower jaw. The premolars and molars are commonly seven, as in the placental mammals, but their arrangement is reversed, as there are four true molars and three premolars ; and finally the milk dentition of all known Marsupials, existing or extinct, is (if not entirely absent) limited to a single tooth on each side of each jaw, this being the predecessor of the last permanent premolar. In very few mammals are teeth entirely absent. Even in the Whalebone Whales their germs are formed in the same manner and at the same period of life as in other mammals, and even become partially calcified, but they never rise above the gums, and completely disappear before the birth of the animal. In some species of the order Edentata, the true Anteaters and the Pangolins, no traces of teeth have been found at any age. The Monotremata are in like case, although the Ornithorhynchus has flattened, ridged, horny plates at the back of both jaws, which answer the purpose of molar teeth. Modifications of the Teeth in Relation to their Functions. The principal functional modifications noticed in the dentition of Mammalia may be roughly grouped as (1) piscivorous, (2) carnivorous, (3) insectivorous, (4) omni vorous, and (5) herbivorous, each having, of course, numerous variations and transitional conditions. Pisci- 1. The essential characters of a piscivorous dentition are vorous best exemplified in the Dolphins, and also (as modifications lon - of the carnivorous type) in the Seals. It consists of an 1 Hyrax alone among existing mammals which have four premolars has also four milk molars. elongated, rather narrow mouth, wide gape, with numerous subequal, conical, sharp-pointed, recurved teeth adapted simply to rapidly seize, but not to divide or masticate, active, slippery, but not powerful prey. All animals which feed on fish as a rule swallow and digest them entire, a process which the structure of prey of this nature, especi ally the intimate interblending of delicate, sharp-pointed bones with the muscles, renders very advantageous, and for which the above-described type of dentition is best adapted. 2. The carnivorous type of dentition is shown in its Cam most perfect development among existing mammals in vor o&amp;lt; the. Eelidee. The function being here to seize and kill struggling animals, often of large size and great muscular power, the canines are immensely developed, trenchant, and piercing, and are situated wide apart so as to give the firmest hold when fixed in the victim s body. The jaws are as short as is consistent with the free action of the canines, so that no power may be lost. The incisors are very small, so as not to interfere with the penetrating action of the canines, and the crowns of the molar series are reduced to scissor-like blades, with which to pare oft the soft tissues from the large bones, or to divide into small pieces the less dense portions of the bone for the sake of nutriment afforded by the blood and marrow it contains. The gradual modification between this and the two following types will be noticed in their appropriate places. 3. In the most typical insectivorous animals, as the insei Hedgehogs and Shrews, the central incisors are elongated, voro pointed, and project forwards, those of the upper and lower jaw meeting like the blades of a pair of forceps, so as readily to secure small active prey, quick to elude capture, but powerless to resist when once seized. The crowns of the molars are covered with numerous sharp edges and points, which working against each other, rapidly cut up the hard cased insects into little pieces, fit for swallowing and digestion. 4. The omnivorous type, especially that adapted for the Omn consumption of soft vegetable substances, such as fruits of vor various kinds, may be exemplified in the dentition of Man, of most Monkeys, and of the less modified Pigs. The incisors are moderate, subequal, and cutting. If the canines are enlarged, it is usually for other purposes than those connected with food, and only in the male sex. The molars have their crowns broad, flattened, and elevated into rounded tubercles. 5. In the most typically herbivorous forms of dentition, Hert as seen in the Horse and Kangaroo, the incisors are well vor developed and trenchant, adapted for cutting off the herbage on which the animals feed ; the canines are rudi mentary or suppressed ; the molars are large, with broad crowns, which in the simplest forms have strong transverse ridges, but may become variously complicated in the higher degrees of modification which this type of tooth assumes. The natural groups of mammals, or those which in our present state of knowledge we have reason to believe are truly related to each other, may each contain examples of more than one of these modifications. Thus the Primates have both omnivorous and insectivorous forms. The Carnivora show piscivorous, carnivorous, insectivorous, and omnivorous modifications of their common type of denti tion. The Ungulata and the Rodentia have among them the omnivorous and various modifications, both simple and complex, of the herbivorous type. The Marsupialia exhibit examples of all forms, except the purely piscivorous. Other orders, more restricted in number or in habits, as the Proboscidea and Cetacea, naturally do not show so great a variety in the dental structure of their members. In considering the taxonomic value to be assigned to