BRANCHIURA FROM LAKE TANGANYIKA. 



267 



prominent in some individuals than others. An extensive series 

 of measurements undertaken by the writer has shown that, while 

 sizes and proportions may be stated in general terms, a con- 

 siderable degree of variation is commonly met with, and that 

 young specimens as a rule deviate somewhat from the normal 

 specific characteristics. 



Concerning the actual figures given in the specific descriptions, 

 it is to be noted that the proportions are not always precisely 

 those adopted by Wilson. The latter gives the relative length of 

 the abdomen as a fraction of the entire length of the animal 

 exclusive of the abdomen. This has seemed to me less satisfactory 

 than to express it as a fraction of the total length of the animal. 

 What is wanted is to indicate briefly what proportion of the total 

 length is constituted by the abdomen, and this mode of expression 

 has therefore been chosen in the following descriptions. 



Again, in the matter of the suckers it seems likely that Wilson's 

 fractions are not strictly comparable with those given in this 

 paper, although the same plan has been adopted of expressing the 

 size as a proportion of the breadth of the cai-apace. My earlier 

 sucker measurements were all made on a basis of the over-all 

 diameter contrasted with the carapace breadth. They gave 

 decimal fractions which seemed to vary somewhat eri-atically, 

 and it was, moreover, most difficult to decide what might be 

 legitimately considered as the edge of the sucker in an average 

 state of expansion. The well-marked chitinous ring to which 

 the membranous border of the sucker is attached seemed to offer 

 a much more definite structure for investigation, the dimensions 

 of which could be clearly observed. A further series of measure- 

 ments, in which the size of this chitin ring was taken as a 

 standard of comparison, gave results, when expressed in fractions, 

 which were distinctly more uniform ; and this method has con- 

 sequently been adopted for arriving at the figures given in the 

 specific descriptions. The diameter of the suckers and the 

 corresponding decimal fractions are both smaller, of course, than 

 if the over-all dimensions had been taken ; and as the figures 

 obtained are smaller, on the whole, than those given by Wilson, 

 it appears highly probable that his are based on the maximum 

 diameter of the suckers. 



Key to the African Species q/ Argulus. 



a. Carapace orbicular, about as long as broad. 



h. Antero-lateral depressions very pronounced ; no accessory 

 spines or processes behind the antennules and maxil- 



lipeds incisns. 



b'. Antero-lateral depressions slight ; accessory spines present 



behind the antennules and maxillipeds ruhrofimctatus S • 



a'. Carapace elliptical, distinctly longer than broad. 

 b. Basal segment of maxilliped bearing spines. 



c. Second segment of anteunule with anterior hook. 

 d. Abdomen long elliptical, lobes bluntly pointed and 



separated by moderately broad sinus personatus. 



d' . Abdomen broadly elliptical, lobes obtusely pointed 

 and separated bj' broad sinus ; carapace with char- 

 acteristic reddish spots rubropunctat as 9 • 



18* 



