8 SKETCH OF THE HISTORY OF MAMMALIA. 



each jaw (if we except the hares and rabbits, in which 

 two minute incisors rise at the back of the large perma- 

 nent ones) : these are strong, compressed, and somewhat 

 curved, with sharp chisel-shaped edges. It is only their 

 anterior surface that is covered with a thick layer ot 

 enamel, and this layer forms the cutting edge, as does 

 the layer of steel on softer metal composing a common 

 chisel. Their insertion into their sockets is very deep, 

 but the inserted part is not a true root : these incisors 

 spring from a pulpy germ at their base, from which they 

 are perpetually growing, and this growth bears a due 

 proportion to the rapidity with which their cutting edges 

 wear away by use. So imperative is this law, that, where 

 one incisor is lost by accident, its opposite, hving no 

 countercheck, keeps increasing, till it acquires an enor- 

 mous development, to the annoyance, and often the de- 

 struction, of the sufferer. With regard to the molars it 

 may be observed that they differ in number in different 

 species : they are, however, generally characterised by 

 a flat surface ; traversed transversely by ridges of enamel, 

 their structure being composed of perpendicular folds of 

 this substance, compacted together by intervening osseous 

 matter ; but further than this, we find in different spe- 

 cies a structural distinction of physiological importance : 

 in some, as the Arvicolidce, they resemble the incisors, 

 having no true solid roots, but are perpetually growing 

 *as their surface wears away ; in others, on the contrary 

 (as the squirrels), at a certain period they gain truly 

 formed roots, and after this cease all further growth. 

 In the Rodentia the upper lip, which is cleft longitudi- 

 nally, is in many species an organ of prehension ; or at 

 least is of great importance in gradually transmitting the 

 food into the mouth, as may be seen when we offer the 

 rabbit a leaf or a stalk of clover or dandelion. The pha- 

 rynx, or back of the mouth, is contracted, and in some 

 species funnel-shaped, and capable of being closed by a 

 circular muscle, in order that the food may pass gradu- 

 ally, as it becomes duly ground to pulp between the 

 molars. The structural organisation of the Rodents, as 

 evidenced by the characters of the skull, the bird-like 



