DISTINCTIVE CHARACTERS OF THE CRAYFISH. 241 
longest: and when the latter are spread out at right 
angles to the body, the distance from tip to tip of the dac- 
tylopodites is equal to, or rather greater than, the extreme 
length of the body from the apex of the rostrum to the 
posterior edge of the telson, in both sexes. In both sexes, 
the length of the swimmerets hardly exceeds half the trans- 
verse diameter of the somites to which they are attached. 
The exopodites of the appendages of the sixth abdo- 
minal somite (the extreme length of which is rather 
greater than that of the telson) are divided into a larger 
proximal, and a smaller distal portion (fig. 87, F, p. 144). 
The latter is about half as long as the former, and has a 
rounded free edge, setose like that of the telson. There 
is a complete flexible hinge between the two portions, 
and the overlapping free edge of the proximal portion, 
which is slightly concave, is beset with conical spines, 
the outermost of which are the longest. The endopodite 
has a spine at the junction of its outer straight edge 
with the terminal setose convex edge. A faintly marked 
longitudinal median ridge, or keel, ends close to the 
margin in a minute spine. The tergal distal edge of 
the protopodite is deeply bilobed, and the inner lobe 
ends in two spines, while the outer, shorter and broader 
lobe, is minutely serrated. 
In addition to the characters distinctive of sex, which 
have already been fully detailed (pp. 7, 20, and 145), there . 
is a marked difference in the form of the sterna of the three 
posterior thoracic somites between the males and females. 
k 
