308 DEVELOPMENT. 



complex crowns than tlieir predecessors(l). The bicuspides, in 

 Human Anatomy, and the corresponding teeth, called ' premolars/ 

 in the lower Mammals, illustrate this law. 



The first true molar owes the germ of its matrix to a vege- 

 tation or bud, separated by the fissiparous process from the matrix of 

 a pre-existing 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 horizon- 

 tally. In this successive germ- production we find repeated the muci- 

 parous property of the dental matrix of the Crocodile ; but the con- 

 comitant growth of the jaw allows the second, third, and sometimes 

 fourth generation 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 repro- 

 duction of horizontally succeeding teeth stops at the third gene- 

 ration ; 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 hori- 

 zontally succeeding molars(2) ; and in most of the species the 

 four true molars are in use and place at the same time ; but in the 

 Kangaroos the anterior ones are shed before the posterior ones 

 are developed. This successive decadence is still more characteristic 

 of the grinding teeth of the Elephant, which consist exclusively of 

 true molars. 



(1) " C'est une regie generale, que les molaires de remplacement ont une couronne 

 moins compliquee que celles auxquelles elles succedent; mais cette couronne compliquee 

 se trouve reportee sur les molaires permanentes qui viennent plus en arriere." This gene- 

 ralization was established by Cuvier. Legons d'Anat. Comp. ed. 1805, vol iii. p- 135. 



(2) 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 of persistence of 

 the same power in the vertically succeeding teeth of the cold-blooded Ovipara, and is associated 

 with many other instances of the same afRnity in more important parts of the organization of 

 the implacental Mammals. 



