THE TEETH OF RODENTIA. 367 



not been found in any of the group, save in the hares and 

 rabbits. 



In the hares and rabbits there are four incisors jnjjje 

 upper jaw, a small and apparently functionless pair being 

 placed close behind the large rodent incisors ; but in very 

 young specimens there are six incisors, of which the one pair 

 are soon lost. 



Prof. Huxley (Nature, vol. 23, p. 228) has recently written 

 that " the deciduous molars and the posterior deciduous 

 upper incisors of the rabbit have been long known. But I 

 have recently found that unborn rabbits possess, in addi- 

 tion, two anterior upper and two lower deciduous incisors, 

 Both are simple conical teeth, the sacs of which are merely 

 embedded in the gum. The upper is not more than one- 

 hundredth of an inch long, the lower rather larger. It 

 would be interesting to examine frctal guinea-pigs in rela- 

 tion to this point ; at present they are known to possess 

 only the hindmost deciduous molars, so far agreeing with 

 the Marsupials." 



Hares and rabbits have six milk molars in the upper and 

 four in the lower jaw, which come into use, but differ from 

 their successors in forming definite roots and not growing 

 from persistent pulps. 



Other rodents, such as the rat, which has only three 

 teeth of the molar series on each side, and the Australian 

 water-rat ( Hydro mys) have no milk teeth, and are hence 

 truly Monophyodont. 



More diversity exists in the premolar and molar teeth ; 

 in rodents of mixed diet, such as the common rat, the back 

 teeth are coated over the crown with enamel, which nowhere 

 forms deep folds, and have distinct roots, i.e., are not of 

 persistent growth ; the molars of the rat have some sort of 

 resemblance to minute human molars. In aged specimens 

 the enamel is consequently worn off the grinding surface of 



