RODENTIA. 



385 



osteum, which covered the portion of bone 

 absorbed from the bottom of the alveolus. 

 The jumping hare (Hclamys capensis), when 

 full grown, offers a good example of this cu- 

 rious structure. 



The molars are not numerous in any Ro- 

 dents ; the hare and rabbit (Lepus) have 



- - ; i. e. six molars on each side of the upper 

 5 5 



jaw, and five on each side of the lower jaw : 

 the pika (Lagoniys) > has - - ; the squirrels 



55 



55 



have j- -r ; the families of the dormice, the 



porcupines, the spring rats (Echingidce), the 

 octodonts, chinchillas, and cavies, have 

 4 4 

 -JT molars ; in the great family of rats 



, the normal number of molars is 



q o 



; but the Australian water rat (Hydro- 



2 2 



mys) 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 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 addi- 

 tional ones are anterior to these, and are pre- 

 molars, i. e. they have each displaced a deci- 

 duous predecessor in the vertical direction, 

 and are what Cuvier calls dents de remplacc- 

 ment. This it is which constitutes the 

 essential distinction between the dentition 

 of the marsupial and the placental Rodent ; 

 the latter, like the placental Carnivora, Ru- 

 minantia, and ordinary Pachydermata 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 pre-molar, 

 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 Cu- 

 vier in the Guinea-pig. 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, de- 

 ciduous 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 pre-molars of the Guinea-pig, by uterine 

 teeth ; but the second incisor was observed by 

 Cuvier to be so preceded in the genus Lepus, 

 and he has figured the jaw of a young rabbit, 

 before that deciduous tooth was shed, when 

 six incisors are present in the upper jaw. 

 This condition is interesting, both as a transi- 

 tory manifestation of the normal number of 



VOL. IV. 



incisive teeth in the Mammalia series, and as 

 it elucidates the disputed nature of the great 

 anterior scalpriform teeth. GeofFroy St. 

 Hilaire contended that the scalpriform teeth of 

 the Rodents were canines, because those of the 

 upper jaw extended their fang backwards into 

 the maxillary bone, which lodged part of their 

 hollow base and matrix. But the scalpriform 

 teeth are confined exclusively to the inter- 

 maxillary bones at the beginning of their 

 formation; and the smaller incisors, which are 

 developed behind them in our anomalous 

 native Rodents, the hare and rabbit, retain 

 their usual relations with the intermaxillaries, 

 and, a fortiori, prove the tooth which projects 

 anterior to them to be also an incisor. 



The law of the unlimited growth of the scal- 

 priform incisors is unconditional, and constant 

 exercise and abrasion are required to maintain 

 the normal and serviceable form and propor- 

 tions of these teeth. When, by accident, an op- 

 posing incisor is lost, or when by the distorted 

 union of a broken jaw the lower incisors no 

 longer meet the upper ones, as sometimes hap- 

 pens to a wounded hare, the incisors continue 

 to grow until they project like the tusks of the 

 elephant, and the extremities, in the poor 

 animal's abortive attempts to acquire food, also 

 become pointed like tusks : following the 

 curve prescribed to their growth by the form 

 of their socket, their points often return against 

 some part of the head, are pressed through the 

 skin, then cause absorption of the jaw-bone, 

 and again enter the mouth ; rendering masti- 

 cation impracticable, and causing death by 

 starvation. In the Museum of the College of 



Fig. 269. 



Incisor teeth of the upper jaw of a Rabbit, showing 

 the effects of unchecked 'growth on the scalpriform 

 incisor (i), and the accessory incisor (i, 2). 



Surgeons there is a lower jaw of a beaver in 

 which the scalpriform incisor has,by unchecked 

 growth, described a complete circle ; the point 

 has pierced the masseter muscle and entered 

 the back of the mouth, passing between the 

 condyloid and coronoid processes of the 

 lower jaw, descending to the back part of the 

 molar teeth, in advance of the part of its own 

 alveolus, which contains its hollow root. The 

 upper jaw of a rabbit with an analogous 

 abnormal growth of the scalpriform and acces- 

 sory incisors is represented in fig. 269. 



Organs of digestion. The order of Rodent 

 quadrupeds comprehends animals which are 

 nourished by various kinds of food, both 

 animal and vegetable substances forming the 

 nutriment of some genera, whilst others live 

 exclusively upon the fruit, bark, or leaves of 



c c 



