100 NATUEAL HISTORT OF 



commonly quadrate, usually wider than long, and 

 sometimes rounded or triangular. It is of a horny 

 consistence, and its direction is often somewhat 

 slanting or vertical. Its outer margin is frequently 

 sinuated or notched. 



The mandibles, or upper jaws, are two strong 

 horny pieces, which take their rise one from each 

 side of the mouth, immediately beneath the upper 

 lip, which usually covers their base. They are ge- 

 nerally of a triangular form, more or less curved, 

 esp cially towards the tip, and usually toothed or 

 serrated on their inner edges. They may be said 

 to r present the jaws in vertebrate animals, but they 

 differ in this, that their motion, instead of being ver- 

 tical, is horizontal or from side to side. Their in- 

 ternal surfaces are frequently parallel, but the den- 

 tati ns are seldom alike in both, the projections of 

 the one being so arranged as to enter the concavi- 

 ties of the other, in order to admit of their closer 

 junction. This, however, is often prevented by 

 the curvature of the tips ; and in several instances 

 where the mouth is wide, and the mandibles rather 

 remote from each other at the base, the blades cross 

 each other a little beyond the centre. The name, 

 which is derived from a Latin word signifying to 

 chew, indicates their function, which is to bruise 

 and comminute the food ; and they likewise act as 

 instruments of prehension. They are liable to re- 

 markable variation in size : in many genera they 

 v e so short and inconspicuous as to be almost en- 



