DISTINCTIVE CHARACTERS OF THE CRAYFISH. 237 
The posterior edges of these ridges are continuous with 
the labrum. The anterior margin is produced in the 
middle into a fleur de lys shaped process, the summit 
of which terminates between the antennules. At the 
sides of this process, the anterior margin of the epis- 
toma is deeply excavated to receive the basal joints of 
the antenne. Following the contours of these excavated 
margins, the surface of the epistoma presents two lateral 
convexities. The widest and most prominent part of each 
of these lies towards the outer edge of the epistoma, 
and is produced into a conical. spine. Sometimes 
there is a second smaller spine beside the principal one. 
Between the two convexities les a triangular median 
depressed area. 
The distance from the apex of the anterior median 
process to the posterior ridge is equal to a little more 
than half the width of the epistoma. 
The corneal surface of the eye is transversely elongated 
and reniform, and its pigment is black. The eye-stalks 
are much broader at their bases than at their corneal 
ends (fig. 48, A). The antennules are about twice as long 
as the rostrum. The tergal surface of the trihedral 
basal joint of the antennule, on which the eye-stalk rests, 
is concave; the outer surface is convex, the. inner flat 
(figs. 26, A, and 48, B). Near the anterior end of the 
sternal edge which separates the two latter faces, there 
is a strong curved spine directed forwards (fig. 48, B, a). 
When the setz, which proceed from the outer edge of 
