364 A MANUAL OF DENTAL ANATOMY. 



decussation of the prisms which is very similar to that de- 

 scribed by my father as characteristic of all the Rodents 

 save the Leporidse (Hares) and Hystricidae (Porcupines). 



THE TEETH OF RODENTIA. 



The animals belonging to this order, which is sharply 

 denned, are scattered almost all over the world ; the island 

 of Madagascar is, however, remarkable for being almost 

 without indigenous Rodents, as is the case also with Aus 

 tralia, two facts which are of no small interest to the 

 student of odontology. 



For in each of these areas, out of the creatures which are 

 there (in the one Lemurs, in the other Marsupials), there 

 has arisen a form so modified as to mimic the dentition of 

 the true Rodents, viz., the Cheiromys in Madagascar, and 

 the Wombat in Australia. 



The species of Rodents are exceedingly numerous, and 

 the great majority of them are of small size ; the aquatic 

 Capybara is far the largest of recent Rodents. 



In general features the dentitions of the numerous 

 species comprising this order are very uniform ; the incisors, 

 (save in the hares and rabbits, in which there is an ac- 

 cessory small pair immediately behind the large ones) are 

 reduced to four in number, are of very large size, and grow 

 from persistent pulps. The jaws for some little distance 

 behind the incisors are devoid of teeth, while beyond the 

 interval the back teeth, generally not more than four in 

 number, are arranged in lines which diverge slightly as they 

 pass backward. The large scalpriform, or chisel-like incisors, 

 extend far back into the jaws, and are much curved, the 

 upper incisors, in the words of Professor Owen, forming a 

 larger segment of a smaller circle than the lower, which are 

 less curved. The length and curvature of these incisors 



