The Squirrel Tribe. 7 



accident one of the incisor teeth has been broken 

 short off. The corresponding tooth in the other jaw, 

 finding no resistance to its growth, continues to in- 

 crease until it sometimes forms a perfect circle outside 

 the mouth, usually resulting in the death of its owner 

 by preventing it from feeding. 



Without some means, however, of preserving the 

 chisel-like sharpness of these teeth, the mere replace- 

 ment of wasted substance would be of little use. In 

 order to obtain the desired result, the teeth are con- 

 structed after a very singular fashion. 



The body of the tooth is composed of pure ivory, 

 coated on the outer surface with a thin layer of 

 enamel, which being of a very much harder nature, 

 is not worn away with the same rapidity. Besides 

 this, the ivory nearest the enamel is harder than the 

 rest, and the softer parts being easiest worn down, the 

 edge of the tooth always keeps the same proportions, 

 the actual cuttiug being performed with the edge of 

 the enamel. 



Our carpenter's chisels are constructed on exactly 

 the same principle, the chief portion of them being 

 composed of soft iron, while a very thin plate of steel 

 is laid along the back and forms the cutting edge of 

 the tool. 



So much for the characteristics of the rodents as a 

 whole. Now for those of that group which are 

 known as squirrels. 



The true squirrels are scientifically known as Sciu- 

 ridce, or shadow-tails, a title derived from two Greek 

 words, the former signifying a shade and the latter a 

 tail. This refers to the habit in these animals of 

 carrying the tail over the back, as though to protect 

 the body from the rays of the sun, a position always 

 adopted except when running or leaping. They are 

 remarkable among the rodents as possessing particu- 



