SURFACE COPEPODA OF THE GULF OF MANNAR. 197 



This species was widely distributed throughout the regions 

 investigated. Numerous examples were obtained showing 

 the modified tail setae as described by Wolfenden {vide ante, 

 p. 193), but, as I have already pointed out, I do not consider 

 this condition to be a true variation. 



Several examples were obtamed which show the double 

 spine on the left side of the posterior thoracic margin. This 

 modification was first described by Giesbrecht (1893, PI. VII., 

 fig. 28). I have since obtained specimens from the east side 

 of the Bay of Bengal, in the neighbourhood of the Andaman 

 Islands and Mergui Archipelago, in which this condition is 

 even more pronounced. In these specimens the left side of 

 the thoracic margin is considerably swollen, and forms a 

 stout prominence projecting outwards and backwards, and 

 bears two stout spines — the dorsal projecting backwards and 

 somewhat upwards, and the ventral pomting verticallj^ 

 downwards and being considerably swollen at its base. 



We thus have three forms, which appear to be quite distinct 

 from one another, for I have up to the present failed to 

 discover any intermediate forms. 



Undinula darwini (Lubbock). 



Calanus darwini, Thompson, 1899, p. 275. 



Cleve, 1901, p. 5. 



A. Scott, 1902, p. 400. 



Thompson & Scott, 1903, p. 241. 



Cleve, 1903, p. 357. 



Cleve, 1904, p. 185. 



Wolfenden, 1905, p. 994. 



Undinula darwini, A. Scott, 1909, p. 17. 



SeweU, 1912, p. 356. 



Pesta, 1913, p. 30, fig. 13. 



This species was much less common in the Ceylon collections 

 than the preceding, though it is of frequent occurrence in 

 other parts of the Indian Ocean. According to Giesbrecht 

 (1893, PI. VII., fig. 29), both first and second abdominal 

 segments are furnished with a row of sharp spines on the 

 2 D 6(10)13 



