SCO 



ANATOMY OF VERTEBRATES. 



the normal number of molars is :; but the Australian Water- 

 rat (Hydromys^ has but -|:-| molars, making, with the incisors, 

 twelve teeth, which is the smallest number in the Rodent order. 

 The greatest number of teeth in the present order is twenty-eight, 

 which is exemplified in the Hare and Rabbit; but forty teeth 

 are developed in these species, ten molars and two incisors being 

 deciduous. 



The first or anterior of the molar series, whether the number 

 be 2-2, 3-3, or 4-4, is a premolar ; it has displaced a deciduous 

 predecessor in the vertical direction. When the series extends 

 to 5-5 or 6-6, the additions are to the fore part, and are pre- 

 molars. This it is which constitutes the essential distinction 

 between the dentition of the marsupial and the placental Rodent ; 

 the latter, like the placental Carnivora, Quadrumana, and Ungu- 

 late*, having never more than three true molars. Thus the Rodents 

 which have the molar formula of -*:-, shed the first tooth in each 

 series, and this is succeeded by a permanent premolar, which comes 

 into place later than the true molars later at least than the first 

 and second, even when the deciduous molar is shed before birth, 

 as was observed by Cuvier in the Guinea-pig. In the Hare and 

 Rabbit, three anterior teeth in the upper jaw, fig 238, p, succeed 

 and displace three deciduous predecessors, ib. d, coming into place 

 after the first and second true molars, ib. m, are in use, and con- 

 temporaneously with the last 

 molar. It does not appear 

 that the scalpriform incisors, 

 ib. z, are preceded by milk 

 teeth, or, like the premolars 

 of the Guinea-pig, by ute- 

 rine teeth ; but the second 

 incisor, ib. z, 2, is so preceded 

 e.g. by the tooth marked 

 d } i, 2, at which period of dentition six incisors are present in the 

 upper jaw. This condition is interesting both as a transitory mani- 

 festation of the normal number of incisive teeth in the mammalian 

 series, and as it elucidates the disputed nature cf the great anterior 

 scalpriform teeth of the Rodentia. It has been contended that 

 they are canines, because those of the upper jaw extended 

 their fang backward into the maxillary bone, which lodged part 

 of their hollow base and matrix. But the scalpriform teeth are 

 confined exclusively to the premaxillary bones at the beginning 

 of their formation, and the smaller incisors which are developed 

 behind them, in our anomalous native Rodents, the Hare and 



Upper deciduous and permanent teeth of the Hare. 



