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152 BULLETIN NO. VIL. 

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 premaxille. The orbits are not shut off from 
the temporal fosse. 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 czeecum 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 

