236 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 
pleura of the sixth somite are narrow, and their posterior 
edges are concave. 
The pits and sete of the cuticle which clothes the 
tergal surfaces of the abdominal somites are so few and 
scattered, that the latter appear almost smooth. In the 
telson, however, especially in its posterior division, the 
markings are coarser and the setz more apparent. 
The telson (fig. 61, G) presents an anterior quadrate divi- 
sion and a posterior half-oval part, the free curved edge 
of which is beset with long sete, and is sometimes slightly 
notched in the middle. The posterior division is freely 
movable upon the anterior, in consequence of the thin- 
ness and pliability of the cuticle along a transverse line 
which joins the postero-external angles of the anterior 
division, each of which is produced into two strong spines, 
of which the outer is the longer. The length of the pos- 
terior division of the telson, measured from the middle 
of the suture, is equal to, or but very little less than, 
that of the anterior division. 
On the under side of the head, the basal joints of the 
antennules are visible, internal to those of the antennz, 
but the attachment of the latter is behind and below 
that of the former (fig. 8, A). Behind these, and in 
front of the mouth, the epistoma (fig. 39, A, II, III) 
presents a broad area of a pentagonal form. The pos- 
terior boundary of this area is formed by two thickened 
transverse ridges, which meet on the middle line at a 
very open angle, the apex of which is turned forwards. 
