DEVELOPMENT. 307 



The matrix of certain teeth does not give rise during any 

 period of their formation to the germ of a second tooth, destined 

 to succeed the first ; this, therefore, when completed and worn 

 down, is not replaced : all the true Cetacea are limited to this 

 simple provision of teeth. In the Armadillos, Megatherioids, and 

 Sloths, the want of germinative power, as it may be called, in the 

 matrix is compensated by its persistence, and by the uninter- 

 rupted growth of the teeth. In other Maramaha the matrix of 

 the first developed tooth gives origin, as described in the Intro- 

 duction, 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 down- 

 wards 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 considerable 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 temporary 

 teeth were adapted to those of the young animal. The perma- 

 nent teeth, which assume places not previously occupied by de- 

 ciduous ones, are always the most posterior in their position, and 

 generally the most complex in their form. 



The successors of the deciduous incisors and canines difi'er 

 from them chiefly in size ; the successors of the deciduous molars 

 may differ likewise in shape ; in which case they have always less 



invests the rest of the fangs, and could not have existed if the dentinal pulp had been 

 detached from the bottom of the capsule in the interspaces of the fangs. The elongation 

 of the pulpy pedicles by the mechanical constriction of the surrounding bone, is an hypo- 

 thetical cause founded on the same imperfect conception of the vital, and especially the absor- 

 bent forces, as that which led Cuvier to ascribe the destruction of the conical base of the 

 Crocodile's old tooth to the mechanical pressure exerted upon it by the apex of the new 

 tooth. The primary branches of the artery of the dentinal pulp correspond in number with 

 the future fangs, at least when the formation of these has begun ; but this condition of the 

 supplying channels may be more justly regarded as a concomitant effect of the modified 

 assimilative and formative processes to which the formation of the fangs is due, than as 

 the efficient cause of such changes in the progressive calcification of the pulp. 



X 2 



