238 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 
the auditory aperture and hide it, are removed, it is 
seen. to be a wide, somewhat triangular cleft, 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 antenne extend as 
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 (F'rontis- 
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 sete, 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 into 
a strong spine. When the abdomen is straightened out, 
if the antenne 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- 
