932 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 
to its extremity, is greater than half the distance from 
the orbit to the cervical groove. It is trihedral in sec- 
tion, and its free end is slightly curved upwards (fig. 41). 
It gradually becomes narrower for about three-fourths of 
its whole length. At this point it has rather less than 
half the width which it has at its base (fig. 61, A); and its 
raised, granular and sometimes distinctly serrated margins 
are produced into two obliquely directed spines, one on 
each side. Beyond these, the rostrum rapidly narrows 
to a fine point; and this part of the rostrum is equal in 
length to the width between the two spines. 
The tergal surface of the rostrum is flattened and 
slightly excavated from side to side, except in its an- 
terior half, where it presents a granular or finely ser- 
rated median ridge, which gradually passes into a low 
elevation in the posterior half, and, as such, may gener- 
ally be traced on to the cephalic region of the carapace. 
The inclined sides of the rostrum meet ventrally in a 
sharp edge, convex from before backwards ; the posterior 
half of this edge gives rise to a small, usually bifurcated, 
spine, which descends between the eye-stalks (fig. 41). 
The raised and granulated lateral margins of the rostrum 
are continued back on to the carapace for a short distance, 
as two linear ridges (fig. 61, A). Parallel with each of 
these ridges, and close to it, there is another longitudinal 
elevation (a, 0), the anterior end of which is raised into a 
prominent spine (a), which is situated immediately behind 
the orbit, and may, therefore, be termed the post-orbital 
