GKXERAL CHARACTERS. MAMMAI.fA.- 



JEXERAL CHARACTERS. 



margins. The two halves of the lower jaw consist 

 each of a single bone ; they are united in front either 

 by a cartilage or by a suture, or sometimes, as in 

 man, the two sides of the jaw are completely amal- 

 gamated so as to form one bony piece. The lower jaw 

 in the Mammalia is articulated directly to the skull, 

 without the intervention of any other movable bone. 



The jaws, as already intimated, are furnished with 

 teeth, and these exhibit a great diversity in their form 

 and structure. They are always implanted in sockets 

 of the jaws, and these are lined by a delicate mem- 

 brane, so that the teeth are never anchylosed or com- 

 pletely united to the bone of the jaws. The teeth 

 consist of a hard substance called dentine, defended by 

 a coating of enamel, and covered by a layer of a third 

 substance called cement. The latter is vry thin on the 

 crown or exposed portion of the tooth in man and 

 many animals, which have teeth similar to those of the 

 human species ; but in the teeth of many herbivorous 

 mammals- the cement acquires a great development, 

 and vertical folds of this substance and enamel penetrate 

 the dentine of the crown, thus giving rise, as the teeth 

 are worn away, to an uneven surface eminently adapted 

 to the comminution of tough vegetable matters. 



A few species are entirely destitute of teeth; in 

 others a few of the teeth are wanting, or some of 

 them undergo peculiar modifications to adapt them to 

 particular purposes. But in the majority we find four 

 dilferent sets of teeth called respectively the incisors, 

 or cutting teeth ; the canines ; the premolars, or false 

 molars ; and the molars, or grinders. The incisors or 

 cutting teeth are inserted in the intermaxillary bones 

 in the upper jaw, and occupy the corresponding place 

 in the lower one. Their number varies from two to 

 ten, and their form is also subject to much diversity; 

 but they are usually flattened transversely, so as to 

 form a cutting edge across the front of each jaw. 



The canines, se called from their large size in the 

 dog, are also very large in all carnivorous mammals. 

 In the human subject the upper ones are frequently 

 called eye-teeth, from their being placed directly 

 beneath the eyes. Of the canines we find one on each 

 side in each jaw ; the upper ones are inserted at the 

 anterior angles of the maxillary bones, and the lower 

 ones in a corresponding position in the lower jaw. When 

 most largely developed, they form long, curved, conical, 

 acute teeth, capable of inflicting the most serious 

 wounds. 



The premolars, which are usually three or four in 

 number on each side, are generally separated by a 

 short interval from the canines, which they frequently 

 resemble in having only a single root ; their crown is 

 usually broad and tubercular or ridged, in a manner 

 more or less resembling that of the true molars. The 

 latter, of which there are also commonly three or four 

 on each side, are the largest and strongest of all the 

 teeth, and are implanted in the jaws by two or more 

 roots, a character peculiar to the Mammalia, and one 

 which is often of the greatest importance to the 

 palaeontologist in determining the nature of those fossil 

 remains by which a certain light has been thrown upon 

 the former history of our planet. The molars, of all 

 the teeth, are those which appear to undergo the 



greatest amount of modification to fit them to the 

 habits and food of the animals. In the carnivorous 

 forms we find them furnished with sharp cutting edges, 

 and fitting together like the blades of a pair of scissors ; 

 in those which prey principally upon insects, whose 

 hard and slippery armour renders them rather difficult 

 to be disposed of, the molars are furnished with a 

 double row of sharp points, from which even the hard- 

 est beetle could not find it easy to escape ; in those 

 which, like the monkeys and our own species, feed 

 upon fruits or upon a mixed diet of soft animal and 

 vegetable substances, the crowns of the molars are of a 

 more or less cubical form, with the surface divided into 

 several blunt tubercles by furrows which traverse it in 

 different directions ; and lastly, the strictly herbivorous 

 species usually present an intermixture or alternation 

 of the three substances of which the teeth are com- 

 posed, such as produces a series of ridges upon their 

 surface, as they are gradually worn down during the 

 trituration of the food. 



The teeth are produced from a pulpy germ or matrix 

 contained within the jaw, and in the majority of the 

 Mammalia the activity of this germ continues after it 

 has served for the formation of the series of teeth first 

 produced. These, which are commonly known as the 

 milk-teeth, are shed at a certain period of life, when 

 their places are taken by new teeth adapted to the 

 increased size of the jaw. The milk-teeth include the 

 incisors, the canines, and three or four molars on each 

 side ; the two former groups are replaced by new 

 incisors and canines ; the deciduous molars are shed 

 to make room for the premolars, whilst the true molars 

 are produced later than the other teeth, and are never 

 changed. The teeth of the Mammalia are never shed 

 more than once; but, in some forms, the formative 

 pulps of some of the permanent teeth continue in 

 activity during the whole life of the animals, and thus 

 the teeth are constantly growing at the root. As these 

 modifications of the teeth are usually characteristic 

 of certain orders of Mammalia, they will be more 

 particularly referred to hereafter, when the beautiful 

 adaptation of their structure to the habits of the ani- 

 mals will be more clearly seen. 



The general structure of the skeleton will not detain 

 us long, as it nearly agrees with that already described 

 (pp. 1, 2), as the most perfect development of the verte- 

 brate type. The vertebral column, or back-bone, as it 

 is usually termed, is divided into several regions, as has 

 been already stated : these are called the cervical, 

 dorsal, lumbar, and sacral regions, or the regions of the 

 neck, back, loins, and sacrum ; and the continuation of 

 the vertebral column into the tail, when this exists, 

 constitutes the caudal region. The same names are 

 applied to the vertebrae composing each region. 



Of the cervical vertebrae there are almost invariably 

 seven ; and this is the only region of the body in which 

 the number of vertebrae is at all constant,* Whatever 

 may be the length of the neck in these animals, the 

 number of the vertebrae is the same ; the short neck 



* The only exceptions to this rule are presented by the 

 Sloths, in which the neck contains eight or nine vertebra : 

 and by the Southern Manatee (Manattu austrcdis}, which 

 usually has only six cervical vertebra. 



