232 THE CUMPARATIVK MORPHOLOGY OF THE CRAYFISH. 



to its extremitj', 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 obliquelj'^ directed spines, one on 

 each side. Beyond these, the rostrum rapidl}^ 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 

 slightl}^ 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 ventralh^ 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 indges, and close to it, there is another longitudinal 

 elevation {a, h), the anterior end of which is raised into a 

 prominent spine (ff), which is situated immediately behind 

 tlie orbit, and may, therefore, be termed the post-orhital 



