[ 542 ] 



Description of a New Species of Brackish-water 

 Gammarus {G. cheureuxi, n. sp.). 



By 

 E. W. Sexton. 



With Five Figures in the Text. 



The Gammarus here described is found in Chelson Meadow, a large 

 tract of reclaimed land lying near the mouth of the Plym, and 

 protected from the tidal water by an embankment. The land is 

 drained by ditches, which empty into the Plym at low water by means 

 of sluice-gates. The sea-water enters in volume only at the highest 

 tides over the top of the sluice-gates, but there is a constant slight 

 infiltration of salt water through the embankment, 



I have named the species in honour of M. Edouard Chevreux, who 

 has always assisted me most generously with his knowledge of the 

 group as well as with specimens from his collection. 



Gammarus chevreuxi resembles G. locusta more nearly than any of 

 the other species of the genus, but there are certain constant charac- 

 ters, e.g. in the antennae, the fourth sideplates, the third uropods, etc., 

 which seem to me of sufficient specific value to justify its establishment 

 as a separate species. 



DESCPtlPTION. 



Gammarus chevreuxi is a small species, the largest male measuring 

 13 mm. in length, and the largest female 9 mm. 



The animals are delicate in appearance, and so transparent that 

 their internal organs show plainly through the cuticle. 



The cuticle in both sexes has a remarkable sensory armature, that 

 of the pleon in particular. All the pleon-segments are covered with 

 microscopic spinules, longest and densest dorsally. Segments 1 to 4 

 are the most spinose ; the spinules of segments 5 and 6 are shorter 

 and finer. The cuticle of the peraeon is not produced in spinules, but 

 has a surface like a fine file, the head and anterior segments being the 

 smoothest. All over the body are scattered microscopic sensory cleft- 

 tipped hairs, each hair set in a little pocket in the skin, some single, 

 some in rows of four to six. Each peraeon-segment carries one of 

 these rows in the median line ; pleon-segments 4 to 6 each have at 

 least three of these rows in line with the usual spine-clusters charac- 

 teristic of the genus ; and the telson has two rows, one on each side. 



