368 GAMMARIM. 



confounded together ; thirdly, from the misapplication 

 of clear descriptions or figures to different species ; and 

 fourthly, from a total disregard of the rules of nomen- 

 clature on the part of subsequent writers who have 

 endeavoured to revise the names of these animals. 



The genus, even in its restricted state, has need of 

 subdivision with a view to greater facility in the deter- 

 mination of the species, and which has been attempted 

 with greater or less success by various carcinologists. 



M. Milne Edwards, in his classical work upon the 

 Crustacea, after the removal of those species which have 

 the eyes of a circular form, divides the species into 

 A. Those which have the first three segments of the 

 tail straight, and not produced in the middle of the 

 hind margin into a tooth ; these are subdivided into, 

 a t those with spines on the fourth and fifth segments of 

 the tail (G. locusta, fluviatilis, marinus } not of Leach ; 

 Olivii, affinis, &c.) : and, a a, those which have no 

 spines on the hinder portion of the tail (G. pulex, 

 Ermanni, Othonis, pinguis, &c.) : whilst the second di- 

 vision, B., comprises those species which have the third, 

 and generally the first and second also, armed with a 

 strong tooth in the middle of the hind margin of each, 

 G. Sabinii, mucronatus, and appendiculatus. According to 

 this arrangement our common fresh-water species, found 

 in rivulets, belongs to the section A. a., but the only 

 fresh-water species which M. Milne Edwards introduces 

 into that section is regarded by him as identical with 

 the insect figured by Rcesel, which has a row of strong 

 teeth along the back, and which M. Gervais, who first 

 identified the species, described under the name of 

 G. Roeselii. We presume this must be an error on the 

 part of M. Milne Edwards, and are accordingly inclined 

 rather to consider our common English species to be 



