100 
NATURAL HISTORY 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 beneatli the upper 
lip, which usually covers their base. They are ge- 
nerally of a triangular form, more or less curved, 
esp daily 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 adqyt of then* 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- 
in arkable variation in size; in many genera they 
are so short and inconspicuous as to be almost en* 
