DESCRIPTION OF A NEW SPECIES OF GAMMARUS. 



543 



Sideplates rather small; the fourth (Fig. 1) forms one of the dis- 

 tinguishing specific characters. The posterior expansion is produced 

 downwards, but not nearly as much as in G. locusta ; it rounds into the 

 inferior margin, and has two setae inset. 



Fig. 1. — Sideplate 4. i Gammariis chevreuxi, n. sp. x 27. 



Fleon. Hind margin of segments 2 and 3 straight, with a few 

 setules inset, postero-lateral corners produced to a short acute angle. 

 Segments 4 to 6 rounded dorsally, 5 and 6 very short ; the dorsal 

 spine-clusters of these segments have each two divergent spines ; the 

 lateral clusters usually with two or three spines and a few hairs in 

 each cluster. 



Head. Lateral lobes not much produced, truncate, upper angle 

 obtuse, rounded below ; sinus rather deep ; post-antennal angle pro- 

 duced, subacute. 



Ei/cs reniform ; pigment usually black coated with white, but occa- 

 sionally a few individuals are found in a brood with red pigment 

 instead of black. 



Fig. 2. — Antenna 1. 6 G'ainmarus chevreuxi, n. s\k Inner side. x 27. 



Antenna 1 (Fig. 2). The first joint of the peduncle is about equal 

 in length to the second and third tuken together. The number of 

 joints in the tiagella varies with age and sex ; one of the largest males 

 had thirty- five in tlie primary tiagellum and seven in the accessory. 



NEW SKKIES. — VOL. IX. NO. 4. MAUCH, 1913. 



2 N 



