232 THE COMPAKATIVE MORPHOLOGY OF THE CRAYFISH. 



to its extremity, is greater tlian 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 obliquel}^ 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 backwaitls ; 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, b), the anterior end of which is raised into a 

 prominent spine («), which is situated immediately behind 

 the orbit, and may, therefore, be termed the post-orhltal 



