906 CRUSTACEA. 



Some species of Gammarus have the fifth epimeral as large as the 

 fourth, but it is subequally lobed, and does not resemble the form 

 mentioned as occurring in certain Amphithoe. In other species the 

 fourth is much smaller than the fifth, as in a part of Amphithoe. 



There are thus good grounds for subdividing the accepted genera 

 Amphithoe and Gammarus. 



1. The species of Amphithoe with the fifth epimeral not smaller 

 than the fourth, and its posterior lobe very small, and with the poste- 

 rior caudal stylets two-branched, with one branch biuncinate (6th 

 kind), may retain the name Amphithoe. The superior antennae are 

 usually the longest. 



2. The species of the same genus with the fifth epimeral smaller 

 than the fourth and subequally lobed, and with the posterior caudal 

 stylets two-branched, the branches nearly similar and neither of them 

 uncinate (5th kind), we call Iphimedia. The genus so called of 

 Eathke embraces species of this kind ; they generally have the supe- 

 rior antennas shorter than the inferior. The genus Acanthosoma of 

 Owen includes a species of the same group, and is of earlier date ; but 

 the name is unfortunately significant, as the existence of spines on 

 the body is unessential ; and it cannot, therefore, be accepted for the 

 group. 



3. The species having one branch of the posterior caudal stylets 

 rudimentary, and, as in Amphithoe, no appendicular branch to the 

 superior antennae, make a third group; and this is the Melita of 

 Leach, as we propose to sustain it. The character of the hands, men- 

 tioned by Leach, is of common occurrence, but we would not make it 

 essential to the genus. 



4. In the more characteristic group of the genus Gammarus, the 

 posterior caudal stylets have two branches, like those in Iphimedia, 

 the two being subequal and different in the apical setae from those of 

 the first or second pair; they are often much longer than occurs in 

 Iphimedia, though not always so. This division includes the Qam- 

 marus pulex, and may well retain the old name Gammarus. 



5. Another portion of the genus Gammarus has one branch of the 

 posterior caudal stylets rudimentary, as in Melita of the Amphithoe 

 series. These make the genus Mazra, Leach's species being included. 



6. A third portion has the stylets quite simple, with the other cha- 

 racters stated under No. 2, page 904, the terminal branch being short, 

 on a long base, and the apex reflexed, quite unlike the form in 



