TEETH. 



901 



spaces unoccupied by the contracted and 

 divided pulp, and affords, by its periosteum, 

 a surface for the adhesion of the cement or 

 ossified capsule covering the completed part 

 of the tooth. 



The matrix of certain teeth does not give 

 rise during any period of their formation to 

 the germ of a second tooth, destined to suc- 

 ceed the first ; this, therefore, when com- 

 pleted and worn down, is not replaced: all 

 the true Cetacea are limited to this simple 

 provision of teeth. In the Armadillos, Me- 

 gatherioids, and Sloths, the want of genni- 

 native power, as it may be called, in the 

 matrix is compensated by the persistence of 

 the matrix, and by the uninterrupted growth 

 of the teeth. 



In most other Mammalia, the matrix of 

 the first developed tooth gives origin to the 

 germ of a second tooth, which sometimes dis- 

 places, sometimes takes its place by the side 

 of, its predecessor and parent. All those 

 teeth which are displaced by their progeny 

 are called temporary, deciduous, or milk 

 teeth ; the mode and direction in which they 

 are displaced and succeeded, viz., from 

 above downwards in the upper, from below 

 upwards in the lower, jaw ; in both jaws 

 vertically are the same as in the Crocodile ; 

 but the process is never repeated more than 

 once in any mammiferous animal. A con- 

 siderable proportion of the dental series is 

 thus changed; the second, or permanent 

 teeth, having a size and form as suitable to 

 the jaws of the adult, as the displaced tem- 

 porary teeth were adapted to those of the 

 young, animal. The permanent teeth, which 

 assume places not previously occupied by 

 deciduous ones, are always the most poste- 

 rior in their position, and generally the most 

 complex in their form. The successors of 

 the deciduous incisors and canines differ 

 from them chiefly in size ; the successors of 

 the deciduous molars may differ likewise in 

 shape, in which case they have always less 

 complex crowns than their predecessors.* 



The " bicuspids," in Human Anatomy, 

 and the corresponding teeth, called " pre- 

 molars," in the lower Mammals, illustrate 

 this law. 



The first true molar owes the germ of its 

 matrix to a vegetation or bud, separated by 

 the fissiparous process from the matrix of 

 the last deciduous tooth ; but the backward 

 elongation of the jaw affords space for its 

 development by the side of its progenitor, 

 during which process it may in like manner 

 give origin to a second, and this to a third, 

 molar, succeeding each other from before 

 backwards or horizontally. 



In this successive germ-production, we 

 find repeated the multiparous property of 

 the dental matrix of the crocodile; but the 



* " C'est une regie ge'uerale, que les molaires cle 

 remplacement out une couronne moins complique'e 

 que celles auxquelles elles succedent ; mais cette 

 couronne complique'e se trouve reported sur les mo- 

 laires permanentes qui viennent plus en arriere." 

 This generalisation was established by Cuvier, in 

 his Lemons d'Anat. Comp., ed. 1805, vol. iii. p. 135. 



concomitant growth of the jaw allows the 

 second, third, and sometimes fourth genera- 

 tion of true molars to co-exist, and come 

 into place side by side. In the Unguiculate, 

 and most of the Ungulate, species of the 

 placental division of the Mammalian class, 

 the fissiparous reproduction of horizontally 

 succeeding teeth stops at the third genera- 

 tion ; in other words, they have not more 

 than three true molars on each side of the 

 upper and lower jaws. In the Marsupial 

 series, the same process extends to a fourth 

 generation of true or horizontally succeeding 

 molars* ; and in most of the species, the 

 four true molars are in use and place at the 

 same time ; but in certain Kangaroos, the 

 anterior ones are shed before the posterior 

 ones are developed. This successive de- 

 cadence is still more characteristic of the 

 grinding teeth of the Elephant, which are 

 finally reduced to a single molar tooth on 

 each side of both jaws. 



Thus the class Mammalia, in regard to the 

 times of formation and the succession of 

 the teeth, may be divided into two groups : 

 the "Monophyodants"-^ or those that 

 generate a single set of teeth ; and the " Di- 

 phyodonts," or those that generate two sets 

 of teeth. 



The Monophyodonts include the orders 

 Monotremata, Bruta (Edentata, Cuv.), and 

 Cetacea (Cetacea vera, Cuv.) : all the rest of 

 the order are Diphyodonts. In these, the 

 first set of teeth are called the milk or de- 

 ciduous teeth : the second set, the adult or 

 permanent teeth ; although the teeth of this 

 set are for the most part, like those of the 

 first set, of limited growth, contracting to a 

 root or roots, and being shed in greater or 

 less proportion during the life-time of the 

 species ; which life-time, in wild Carnivora 

 and Herbivora, is dependent on, and would 

 seem, indeed, to be determined by, the dura- 

 tion of the adult teeth. 



The particulars of the Monophyodont 

 dentition will be found under the Articles 

 MONOTREMATA, Vol. III. p. 387.; CETACEA, 

 Vol. I. pp. 563. 571.573.; (see my Odonto- 

 graphy, p. 345. pis. 87 91.); and EDEN- 

 TATA, Vol. II. p. 53. ; (see also Odontography, 

 p. 317. pis. 76 80.) Examples of some of the 

 striking modifications of dental structure pre- 

 sented by recent or extinct animals of the order 

 Bruta, are given in figs. 5-t8. and 574-. of the 

 present article. It will be observed that I 

 have qualified the generalisation as regards 

 the Monophyodont character of the Cetacea, 

 by citing only that part of Cuvier's order 

 which he termed " true or carnivorous Ce- 

 tacea." The animals of the order Sirenia 



* This characteristic extension of the reproductive 

 power of the matrices of the true molars in the 

 Marsupials, is an approximation to the peculiar 

 activity and persistence of the same power in the 

 vertically succeeding teeth of the cold-blooded Ovi- 

 para, and is associated with many other instances of 

 the same affinity in more important parts of the 

 organization of the implacental Mammals. 



) fMtas, once ; $!*>, I generate ; dbmif, tooth. 



j tlsj twice ; fiiu and eSous 



3 w 3 



