256 THE COMPARATIVE MORPHOLOGY OE THE CRAYFISH. 



definition of the tribe the statement of the special pecu- 

 liarities of the famil}'. 



Thus, the Potamohiidce are those Astacina in which 

 the podohranchife 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 setse, or the longer setie of the podobranchise 

 hooked, though hooked tubercles occur on the stem and 

 on the laminae of the latter. The coxopoditic setfe are 

 always long and tortuous. 



In the Parastacidce, on the other hand, the podo- 

 branchise 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- 



