RODENTS. 411 



that deciduous tooth was shed, when six incisors are present in the 

 upper jaw.(l) 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 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 ex- 

 tended their fang backwards into the maxillary bone, which lodged part 

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

 fined exclusively to the intermaxillary 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 d fortiori, 

 prove the tooth which projects anterior to them to be also an incisor. 

 The law of the unlimited growth of the scalpriform incisors is 

 unconditional, and constant exercise and abrasion are required to 

 maintain the normal and serviceable form and proportions of these 

 teeth. When, by accident, an opposing incisor is lost, or when by 

 the distorted union of a broken jaw the lower incisors no longer 

 meet the upper ones, as sometimes happens 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 abor- 

 tive 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 mastication imprac- 

 ticable and causing death by starvation. In the Museum of the 

 College of 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 coro- 

 noid 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 analo- 

 gous abnormal growth of the scalpriform and accessory incisors is 

 figured in PI. 104. fig. 7. 



(1) PI. 104, fig. 5. 



