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 antennae. Following the contours of these excavated 

 margins, the surface of the epistoma presents two lateral 

 convexities. The widest and most promuient 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 lies 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 triliedral 

 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 setae, which proceed from the outer edge of 



