410 RODENTS. 



160. Succession. — ^The molars are not numerous in any Rodent ; 

 the Hare and Rabbit {Lepus) have jEf, i. e. six molars on each side of 

 the upper jaw and five on each side of the lower jaw : the Pika 

 (Lagomys) , has J^ : the Squirrels have 4^4 : the families of the Dor- 

 mice, the Porcupines, the Spring-Rats {Echimyidae), the Octodonts, 

 Chinchillas and Cavies, have ^-4 molars : in the great family of Rats 

 (Muridce), the normal number of molars is Ie^: but the Austrahan 

 Water-rat, {Hydromys) , has but |e| 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 thirty-six teeth 

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

 deciduous. 



In all the Rodents, in which the number of molars exceeds three 

 in a series, the additional ones are anterior to these and are pre- 

 molars ; i. e. they have each displaced a deciduous predecessor 

 in the vertical direction, and are what Cuvier calls ' dents de 

 remplacement.' This it is which constitutes the essential distinc- 

 tion between the dentition of the marsupial(l) and the placental 

 Rodent ; the latter, like the placental Carnivora, Ruminantia and or- 

 dinary Pachyderma having never more than three true molars. Thus 

 the Rodents, which have the molar formula of JeJ, 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, PI. 104, fig. 4. In the 

 Hare and Rabbit the three anterior teeth in the upper jaw and the two 

 anterior ones in the lower jaw succeed and displace, in like manner, 

 deciduous predecessors, and come into place after the first and second 

 true molars are in use, and contemporaneously with the last molar. 



It does not appear that the scalpriform incisors are preceded by 

 milk-teeth, or, like the premolars of the Guinea-pig, by uterine teeth : 

 but the second incisor was observed by Cuvier(2) to be so preceded in 

 the genus Lepus, and he has figured the jaw of a young Rabbit, before 



(1) See Dentition of the Wombat, p. 393. 



(2) Ossemens Fossiles, torn, v, pt. i, p. 5. 



