238 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 



the auditory aperture and hide it, are removed, it is 

 seen to be a wide, somewhat triangular qleft, which occu- 

 pies the greater part of the hinder half of the tergal 

 surface of the basal joint (fig. 26, A). 



The exopodites, or squames, of the antennae extend aj 

 far as the apex of the rostrum, or even project beyond it, 

 when they are turned forwards, while they reach to the 

 commencement of the filament of the endopodite (Frontis- 

 piece). The squame is fully twice as long as it is broad, 

 with a general convexity of its tergal and concavity of its 

 sternal surface. The outer edge is straight and thick, the 

 inner, which is fringed with long setae, is convex and thin 

 (fig. 48, C). Where these two edges join in front, the 

 squame is produced into a strong spine. A thick outer 

 portion of the squame is marked off from the thinner 

 inner portion by a longitudinal groove on the tergal side, 

 and by a strong ridge on the sternal side. One or two 

 small spines generally project from the posterior and 

 external angle of the squame ; but they may be very 

 small or absent in individual specimens. Close beneath 

 these, the outer angle of the next joint is produced intc 

 a strong spine. When the abdomen is straightened out, 

 if the antennae are turned back as far as they will go 

 without damage, the ends of their filaments usually reach 

 the tergum of the third somite of the abdomen (Frontis- 

 piece). I have not observed any difference between the 

 sexes in this respect. 



The inner edge of the ischiopodite of the third maxilli- 



