CRAYFISHES 207 



ments and attendant setfe terminate in hooks. The first 

 segment of the pleon has appendages in the male and 

 usually also in the female, those of the four following 

 segments being relatively small ; in the male those of tbe 

 first segment are stiliform, and those of the second segment 

 are always peculiarly modified. The telson is frequently 

 divided by a transverse incomplete hinge. 



To this family three genera are assigned, which belong 

 to the fresh waters of the northern hemisphere only. 



Potamohia, Leach, 1819, meaning 'the creature that 

 lives in a river,' is the genus that has so commonly of late 

 years been called Astacus, The name is often also quite 

 needlessly altered into Potamohius, and that by writers who 

 use the name Gehia unchanged, properly ignoring Risso's 

 pseudo-correction of it into Gehios. In Potamohia the last 

 segment of the trunk carries a pleurobranchia and the two 

 or the three preceding segments have rudiments of the 

 pleurobranchia. According to Mr. Walter Faxon, whose 

 authority on this subject is not likely to be disputed, the 

 English species should be called imllipes (Lereboullet), 

 the Potamohia torrentium (Schrank), and the Potamohia 

 flaviatilis (Auctorum) being distinct. It will be remem- 

 bered that it was on this genus that the celebrated Re- 

 aumur conducted his investigations into what was at the 

 time something of a mystery, namely the exuviation or 

 shedding of the coat of the crustacean. Here too Rathke 

 found materials for studying the development of the em- 

 bryo, unfortunately for the commencement of such a study 

 lighting upon an exceptional group, in w^hich the young 

 enters into liberty in a form not very remote from that of 

 its parents. 



GamhdrKs, Erichson, 1846, has the pleurobranchise 

 entirely suppressed, so far as is known, and the podo- 

 branchia of the fourth pair of trunk-legs has no lamina. 

 The third pair of trunk-legs, and sometimes also the second 

 or the fourth pair, have in the male the third joint provided 

 with a conical, recurved, hook-like process, and in the 

 female the hinder edge of the penultimate sternum of the 

 trunk is elevated into a transverse prominence, on the 



