390 
GAMMARIDiE. 
oblique than the first. The walking legs are subequal ; 
the posterior pair of caudal appendages are much larger 
in the male than in the female, and have the inner 
branches one-fourth or one-third shorter than the outer; 
their margins are ciliated, and each branch terminates 
in a minute but distinct joint. 
The females in this species are invariably one-third, 
or even one-half, smaller than the males, their limbs 
are also shorter and more slender, and the branches of 
the sixth pair of caudal appendages are scarcely half the 
length of those of the male. 
The colour of this species is of yellowish brown, 
varying in depth according to the soil over which the 
streams flow in which they dwell. 
We have found these animals in almost every stream 
that we have looked for them ; they are most common 
in shallow overgrown ditches. Sometimes, as in a held 
in Carmarthenshire, they are so abundant that a single 
dip of the sieve would bring up perhaps a hundred 
specimens, but in Devonshire our experience is, that 
they are not only less abundant but also upon the whole 
smaller. 
The close resemblance of the fresh-water species with 
others that exist in the sea is very curious, the more so 
since the inhabitants of either will die if transferred 
from one to the other. And yet we have seen marine 
Crustacea thrive in fresh-water ditches, that were rendered 
brackish by the sea breaking into them only once or 
twice in a year. 
It may be something more than a coincidence that 
gives to each of our fresh -water forms a representative 
species in the ocean ; the key to which may be suggested 
by the interesting discoveries of Cederstrom, Olofson, 
and Widigren, in the lakes of Vetter and Vener, in 
the south of Sweden, of which an account has been 
