463 Dr. T. Scott on British Copepoda. 



appendage, which takes the place of one of the two long, 

 plumose, terminal setse (see PI. XI. fig. 4), but otherwise the 

 second pair closely resemble the second pair in the female. 

 The fifth pair are smaller than those of the female, but do not 

 appear to differ much otherwise. The abdomen consists of 

 five segments, as in the female, the first being furnished with 

 a pair of minute rudimentary appendages. The operculum 

 on the median dorsal aspect of the last abdominal segment 

 and the furcal joints differ distinctly from the same parts in 

 the female, as shown by the drawings (figs. 5, $ , and 6, S). 

 I have not yet ascertained if one or two ovisacs are carried 

 by the female. 



Type species D : ' Arcythompsonia fairliensis, T. Scott. 

 (PI. XL tigs. 1-6.) 



1899. Cyl'mdropsyllm fairliensis, T. Scott, Seventeenth Annual Report 

 of the Fishery Board for Scotland, pt. iii. p. 258, pi. x. tigs. 11-14, 

 pi. xi. figs. 1-4. 



This Copepod, as already stated, is somewhat similar in its 

 form and structure to Cylindropsyllus, G. S. Brady, and for 

 that reason and because no males had been yet observed it 

 was provisionally ascribed to that genus, even though one or 

 two structural peculiarities were noticed and referred to at the 

 end of the specific description. 



The specimens from which the species was described were 

 collected in pools of brackish water near Fairlie, Firth of 

 Clyde. No others appear to have been discovered till quite 

 recently, when two males and a few females were obtained in 

 a small sample of dredged material collected in Lerwick 

 Harbour, Shetland. An examination of these male specimens 

 has enabled me to confirm the opinion I had previously 

 arrived at from an examination of the females, viz., that the 

 differences then observed might " yet render it necessary to 

 remove this Copepod to another genus." 



These males are found to differ from those of the two 

 species of Cylindropsyllus already described in the entire 

 absence of the peculiar terminal appendages with which the 

 outer branches of the second pair of thoracic legs are 

 furnished, and which appear to be characteristic of the males 

 of that genus. These male specimens from Lerwick also 

 want the appendages which characterize the inner branches 

 of the third pair in the males of the same genus. Jn the 

 species under consideration the third pair in the male is 

 practically similar to that of the female. 



The occurrence of these differences in the male, together 



