256 THE COMPARATIVE MORPHOLOGY OF THE CRAYFISH. 
definition of the tribe the statement of the special pecu- 
liarities of the family. 
Thus, the Potamobiide are those Astacina in which 
the podobranchie of the second, fourth, fifth, and sixth 
thoracic appendages are always provided with a plaited 
lamina, and that of the first is an epipodite devoid of 
branchial filaments. The first abdominal somite invari- 
ably bears appendages in the males, and usually in both 
sexes. In the males these appendages are styliform, and 
those of the second somite are always peculiarly modified. 
The appendages of the four following somites are rela- 
tively small. The telson is very generally divided by a 
transverse incomplete hinge. None of the branchial fila- 
ments are terminated by hooks; nor are any of the 
coxopoditic sete, or the longer setee of the podobranchise 
hooked, though hooked tubercles occur on the stem and 
on the lamine of the latter. The coxopoditic sete are 
always long and tortuous. 
In the Parastacidg, on the other hand, the podo- 
branchie are devoid of more than a rudiment of a 
lamina, though the stem may be alate. The podo- 
branchia of the first maxillipede has the form of an 
epipodite ; but, in almost all cases, it bears a certain 
number of well developed branchial filaments. The first 
abdominal somite possesses no appendages in either sex: 
and the appendages of the four following somites are 
large. The telson is never divided by a transverse hinge. 
More or fewer of the branchial filaments of the podo- 
