662 MRS. E. W. SEXTON ON 



of tliem are ovigerous. The largest Hamburg specimen measures 

 22 mm. 



The body is slender and compressed, the brackish-water speci- 

 mens much more delicate in appearance than the freshwater 

 ones. 



Head (fig. 1) about equal in length to the first two pei'feon-seg- 

 ments, in G. locusta it is distinctly shorter ; lateral lobes obliquely 

 truncate, upper angle usually subacute in large specimens, and 

 obtusely rounded in smaller ones, rounded below, sinus small. 



Side-plates smaller than in G. locusta, about as deep as the corre- 

 sponding segments, 1-4 serrated at the anterior angle. The 4th, 

 which forms one of the distinguishing characters of the species, 

 is as broad as deep, with the posterior expansion short and round- 

 ing into the inferior margin. In G. lociosta the posterior expansion 

 is very deep, with the hind margin straight and much more 

 serrated. 



Fleon-segments 1-3 with the postero -lateral corners acutely 

 produced, with only two or three setules inset on the hind 

 margin. Segments 4-6 each with three groups of spines. The 

 number of spines is very variable : generally speaking, the median 

 group has two spines, and the lateral groups three each set fan- 

 wise, and accompanied as a rule by long fine hairs, more numerous 

 in the freshwater animals ; but many of the larger animals have 

 three s^aines in the median group of the 4th segment, and two 

 in the 5th and 6th, and four spines in the lateral groups of 

 the 4th and 5th segments, and three in those of the 6th. The 

 number varies, however, even in animals of the same size. 

 The spines are longer than in G. locusta, and inset at a rather 

 difierent level ; the dorsal groups are only slightly raised, whereas 

 in G. locusta they are elevated and prominent. 



Eyes large, reniform, very dark ; outer row of ommatidia 

 colourless. 



Antenna 1 not quite half as long as the body, much more 

 setose in the male than in the female. The peduncle is longer 

 than in any other known species of Gatnmarus, nearly equalling 

 the peduncle of ant. 2 in length in the adult, and quite equal to 

 it in the young animal. It is furnished inferiorly with the out- 

 standing clusters of stiff set?e characteristic of the species, some 

 of the setae in each cluster extending far beyond the rest, and 

 graduating in length to the distal end of each joint. The 

 1st joint is longer than the 2nd, but not as long as the 2nd 

 and 3rd taken together. The primary flagellum is 18-jointed 

 in the largest Bremerhaven specimen, with 5 joints in the 

 accessory flagellum ; the largest male from Rauschen had 33 

 joints in the primary, and 8 in the accessory, the smaller 

 animals averaged 23-27 in the one, and 4-6 in the other, 

 Zaddach gives the range as from 25-35 in the primary, and 5-9 

 in the accessory. Each joint in the primary flagellum from about 

 the 5th carries a small stalked sensory filament in addition to the 

 small setaj, and a long seta on alternate joints. In G. locusta the 



