CRAYFISHES 207 
ments and attendant sete terminate in hooks. The first 
seoment 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 the 
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. 
Potamobia, 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 Potamolius, and that by writers who 
use the name Gebia unchanged, properly ignoring Risso’s 
pseudo-correction of it into Gebios. In Potumobia the last 
segment of the trunk carries a pleurobranchia and the two 
or the three preceding segments have rudiments of the 
pleurobranchiz. According to Mr. Walter Faxon, whose 
authority on this subject is not likely to be disputed, the 
English species should be called pallipes (Lereboullet), 
the Potamolia torrentium (Schrank), and the Potamobia 
fluviatilis (Auctorum) being distinct. It will be remem- 
bered that it was on this genus that the celebrated Ré- 
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 which the young 
enters into liberty in a form not very remote from that of 
its parents. 
Cambarus, Erichson, 1846, has the pleurobranchiz 
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 
