904 



CRUSTACEA. 



guished only by the form of the hands, is united by Edwards to 

 Amphithoe. Melita of Leach, based on the hand, the finger closing 

 on the side instead of the edge, is also rejected. Acanthosoma of Owen, 

 and Iphimedia of Rathke, characterized by having the superior an- 

 tennas shortest, have fallen back into Amphithoe. Some of these 

 proposed genera, although instituted on unimportant characters, are 

 still natural groups, and, as we show beyond, should be retained. 



In the Gammarus series, the group Gammarus embraces, in the 

 latest system, all the species with hands to the first and second pairs 

 of legs ; while Alibrotus (Edwards) has no hands to the four anterior 

 legs. LeptocMrus (Zaddach) is a new genus, in which the first pair 

 alone is subchelate ; and Iacliyrocerus (Kroyer) differs from Gam- 

 marus in its elongate front, the antennas being situated on its under 

 side, one pair before the other. The genus Mwra of Leach, charac- 

 terized by having one hand of the second pair very large and stout, 

 and the other quite small, has been restored to Gammarus by Ed- 

 wards. Eusirus of Kroyer scarcely differs from Gammarus : it has 

 the four hands large and subequal, and of somewhat peculiar form, 

 with large epimerals and reniform eyes; and it may be doubted 

 whether it should be sustained. 



In instituting the genera of Gammarinas, certain important sources 

 of distinction have been overlooked. The natural groups have in part 

 been recognised, and have again been rejected for want of the suffi- 

 cient characters which these means afford. These characteristics are 

 found in the caudal stylets, the epimerals, and the organs of the 

 mouth. The caudal stylets, especially the posterior pair, are highly 

 distinctive, affording truthful lines of demarcation among natural 

 groups, although organs seemingly unimportant. That this unimpor- 

 tance is not real, the connexion of these organs with the powers of 

 locomotion in both the Orchestidae and Gammaridae fully evinces. 



We may briefly review the forms in the posterior stylets occurring 

 among these species. 



1. One-branched, the branch very short, straight, with a few setules 

 at apex and rarely elsewhere. This is the only form occurring in the 

 Orchestidae. 



2. One-brancJwd, the branch short and naked or nearly so, on rather 

 a long base, the apex somewhat reflexed, and enclosing two exceed- 

 ingly short but thickish spines, pointing upward or a little inward, 

 and looking as if just emerging from the apex. This form is found 



