390 Annals of the South African Museum. 



behind it. In Koehler's species of Arcturopsis, on the other hand, 

 the 4th segment is greatly elongate, exceeding in length the rest of 

 the body behind it. Opinions may differ as to this being of generic 

 importance, but it seems scarcely necessary to consider it so, for 

 there always remains the possibility of discovering transitional forms. 

 In fact lineata and corniger are examples, the former having the 

 4th segment equal to, the latter a little longer than, the posterior 

 portion of the body. 



Sars' definition must be slightly modified ; flagellum of antenna 2 

 1-3-jointed, with 2 rows of denticles on the lower surface ; 3rd 

 peraeon segment in male with (typically), but sometimes without, 

 a ventral process, when present more or less strongly developed ; outer 

 ramus of pleopod 1 in male not modified. 



In the course of studying the specimens belonging to this genus, I 

 have been confronted in one case with a difficulty similar to that which 

 arose in the case of the genus Cymodoce, namely that of correlating the 

 male and female. The facts were as follows : 



A and 9 were taken from a bottle, P.F. No. 15817, the contents 

 of which were all dredged on the same day and in the same spot. These 

 were the only Astacillids taken in that haul, and would be regarded as 

 conspecific under the usual working hypothesis and unless evidence to 

 the contrary were forthcoming. 



In fact this <$ agrees perfectly in structure with the <$ here 

 assigned to lineata which were taken in association with the 9 9 

 entirely different from the $ from bottle 15817. Since then it is 

 better to presume that the sexes are similar rather than dissimilar, 

 even in a family in which sexual dimorphism is a common phenomenon, 

 the following morphological reason points against the $ and $ from 

 bottle 15817 being couspecific ; the ornamentation of the 4th peraeou 

 segment in the $ $ and 9 $ of lineata, taken in the same haul, 

 consists in both cases of 2 mediodorsal tubercles. 



Key to the South African Species of Arcturella. 



A. Width of 4th peraeon segment in $ less than length, in <$ very 

 much less. Body in both sexes subcyliudrical. 

 Flagellum of antenna 2, 2- or 3-jointed. 



i. A small ventral process on 3rd segment 

 in (J . Body in $ normally hirsute 

 and strongly tuberculate . . corniger (Stebb.). 

 ii. No ventral process. Body in $ glabrous 



and feebly tuberculate . . . lineata (Stebb.). 



