390 GAMMARID^. 



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 field 

 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 



