280 "terra nova" expedition. 



Sub-order MYSIDA. 

 Family MYSIDAB, Dana. 



SuB-PAMiLY SIRIELLINAE, Norman. 

 Genus SIRIELLA, Dana. 



2. Siriella thompnonii (H. Milne-Edw.). 



Occurrence. — North and South Atlantic, Stations 31, 53, 58, 59, 61-65, 67-70. 

 About 200 specimens, mostly immature. 



New Zealand waters in the area bebween Three Kings Islands and the North Cape 

 of New Zealand, Stations 75, 83-86, 89, 92, 93, 100, 101, 106, 108-110, 113, 120, 122, 

 126, 130. About 150 specimens. 



This abundant and widely-distributed species occurred in thirteen hauls in the 

 Atlantic, and nineteen hauls in New Zealand waters. At ten Atlantic stations and 

 twelve New Zealand stations it was captured at the surface. Only one of the remaining 

 hauls was made at a greater depth than 3 metres, namely, at Station 70, off the Azores, 

 where the net was lowered to 12 metres. 



On the journey out, thirty-three hauls of plankton were made in the Atlantic 

 Ocean, all of them during daylight, and S. thompsonii occurred in only one. On the 

 return journey through the Atlantic, thirty-one plankton gatherings were made, five 

 by day, and twenty-six at niglit. S. thompsonii was captured in eleven of the night 

 hauls, but not once by day. 



Similarly, in the New Zealand area, seventy-one plankton gatherings were made, 

 forty-six by day, and twenty-five by night. This species occurred in seven (or 15 per 

 cent.) of the day hauls, and in twelve (or 48 per cent.) of those made during hours of 

 darkness. Moreover, all the gatherings of plankton in which more than ten specimens 

 of S. thompsonii were found, were night hauls. These facts, while showing that this 

 species is a pelagic, oceanic and essentially surface form, would seem to indicate that 

 • it is more abundant at the surface during hours of darkness, and probably descends to 

 greater depths during the daytime. A diurnal movement is thus suggested. 



Distribution. — Very generally distributed in the tropical and sub-tropical waters 

 of the world. 



Previous records of this species for the oceanic waters in the neighbourhood of 

 Australia and New Zealand are given by Sars (1885) for specimens taken by the 

 " Challenger " on the voyage between Sydney and Wellington, and by Colosi (1918 

 and 1920) for specimens caught in Lat. 28° 20' S., Long. 170° 5' E. 



3. Siriella denticulata (G. M. Thomson). 



Mysis denticulata, 6. M. Thomson, 1880, p. 1 ; 1881, p. 205, pi. YII, fig. 6 ; Siriella denticulata, 

 G. M. Thomson, 1900, p. 482, pi. 33, figs. 1-5 ; Hutton, 1904 ; Thomson, 1913 and 1921 ; 

 Tattersall, 1913. 



Occurrence. — North of New Zealand. Station 85, one female, 10 mm. ; Station 93, 



