152 BULLETIN NO. VII. 



Rodents agree in lacking, even in the milk dentition, canine 

 teeth a very important and constant character. The incisors 

 are rootless and grow during the life of the animal. They form 

 segments of a circle, the upper being the smaller circle. The 

 outer surface only is provided with enamel, thus providing 

 for automatic sharpening by mutual attrition. It is interesting 

 to notice a curious adaptation for the use of these chisel- shaped 

 teeth in squirrels, for instance. The symphysis, or line of 

 union of the two lower jaws, is cartilaginous only, permitting 

 the play of the jaws laterally. By this means the teeth when 

 inserted in the nut or acorn shell, are pried apart, splitting the 

 shell with ease. 



The incisors are frequently ornamented with ridges or cor- 

 rugations. The number of molars is not large, never more 

 than six or less than two in one ramus. The number of true 

 molars is three, and these were not preceded by milk teeth 

 as is usually the case with the premolars. The molars are some- 

 times rootless, and at others have well defined crown and roots* 

 but the distinctions based on this circumstance are of subordi- 

 nate worth. These two kinds of molars are found in closely 

 related families, for instance among mice. The variations in 

 the internal structure are so great as to make general state- 

 ments of little value. The skull is generally elongate with 

 well- developed premaxillse. The orbits are not shut off from 

 the temporal fossae. The interparietal is well developed. 

 There is a ninth bone in the carpus. The clavicles are present 

 in the arboreal and fossorial groups, but absent in others. 

 The brain surface is smooth, and the cerebrum extends but 

 slightly backward. The digestive tract conforms to the 

 herbiverous habits of the animals, a large caacum being found 

 in all but the dormice. 



The nails, though generally claw-like, are occasionally mod- 

 ified to closely resemble hoofs ; in some cases, also, the bones 

 of the leg are elongated and the mid-foot bones form a kind of 

 cannon-bone, adapting the possessor to a saltatorial life, and 

 indicating that the ungulates are not so completely distinct 

 from the claw-bearing animals as sometimes suggested. The 

 rodents are extraordinarily fertile, and reproduce with incred- 

 ible rapidity, even though exposed to constant inroads from 

 beasts and birds of prey. The great fecundity becomes in 

 some cases the occasion of distress, and forced migrations, 

 forming an interesting parallel with human history. The 

 lemming of northern Europe has repeatedly sent southward 



