GAMMARIDEA. 907 



Maera. The species constitute our genus Dercothoe. The eye usually 

 occupies a prominence projected forward on the side margin of the 

 head. The fifth epimeral, in the species examined, is as long as the 

 fourth, and subequally lobed. It is probable that all the species have 

 the upper antennae appendiculate. Qammcurus zebra of Kathke* 

 appears to be of this group. The hands of the second pair have a 

 simple finger, and in this respect the genus differs from our Pyctilus, 

 which has the stylets and general habit of Dercothoe. Pyctilus is 

 near Erichthonius of Edwards, but it has the anterior epimerals of 

 moderate size instead of obsolete ; both males and females of this 

 genus were observed, and in each the finger of the second pair of legs 

 was bi-articulate. 



In Kroyer's description of his genus Photis, he mentions that the 

 fifth pair of legs is reversed, the claw being turned backward and 

 very short. We look upon this apparently reversed position of the 

 legs as of small importance. In Amphithoe, as the genus is here 

 restricted, this reversion is common, and it may extend to the sixth 

 and seventh pairs ; in some, the claw is quite reversed, in others it 

 points outward, and in others downward and outward ; these various 

 positions showing that the distinction is not generic. It appears to 

 depend on a slight turning of the fifth joint of the legs, at its articu- 

 lation with the fourth, and may be in part voluntary with the animal. 

 This genus is near Gammarus in its antennas, but has one branch of 

 the posterior caudal stylets rudimentary, and the fifth epimeral as 

 large as the fourth. 



In the following synopsis of the Gammaridae the distinguishing cha- 

 racteristics of the several genera are presented, and remarks are also 

 added on different genera that are passed by without mention in the 

 preceding pages. 



I. Pedes 10 postici non prehensiles. 



Subfam. 1. STEGOCEPHALINiE. — Antennas breves, superiores basi 

 crassse. Mandibulae acie denticulata. instructae, palpo brevi, uniar- 

 ticulato, intus dentato. Epimerae permagnae. 



* Faun, der Norwegens, 74, pi. 3, f. 4. 



