86 DISCOVERY REPORTS 



A much more puzzling problem is raised by the form of the telson in specimens from stations 268, 

 685, 689, 704, 705, 706, 709, 713 and 1602 in this collection. Except for an adult male of 11 mm. 

 from station 1602, in all these specimens, both small ovigerous females (ranging from 4-5 mm. to 

 7 mm.) and adult males (averaging 7 mm. in length) the exopod of the uropod is armed with only two 

 or three spines. In the descriptions of the type and in all figures given for S. thompsonii, the apex of 

 the telson is armed with three pairs of spines with three small median spinules and a pair of plumose 

 setae. These spines are regularly graduated, the outermost pair being shortest and the innermost pair 

 markedly longest. In the specimens from the above-mentioned stations, however, the innermost 

 spines on the telson are shorter than the next pair, often measuring less than half their length. This 

 gives the telson a much more truncate appearance than in the typical thompsonii. Were it not for the 

 large male from station 1602, I would have thought that the three characters of small size, number of 

 spines on the exopod of the uropod and the proportions of the apical spines of the telson were sufficient 

 grounds for the formation of a new species. But this specimen is as large as the largest specimens of 

 thompsonii and the exopods of the uropods are armed with the typical six spines, leaving only the form 

 of the telson as a constant specific character which does not vary with the growth of the animals. Since 

 there is the closest possible agreement with the typical thompsonii in all other respects, I do not feel 

 that there is a valid case for the formation of a new species. All the stations at which these specimens 

 occurred are situated in the West Atlantic, off the coast of South America, from Rio de Janeiro to 

 Pernambuco, and in mid-Atlantic south-west of the Cape Verde Islands. I would suggest that these 

 forms with the short inner pair of apical spines on the telson represent a geographical race of the 

 species S. thompsonii. It may be that further research will reveal forms intermediate between them 

 and the typical members of the species. 



The following notes on colour are given: station 2042: 'Generally colourless and transparent but 

 thoracic region and head blue and purple translucent', and station WS 133 'T': 'Colour semi- 

 translucent blue (sky blue) with lavender antennal scales.' 



Distribution. S. thompsonii is a pelagic oceanic form widely distributed in the upper layers of 

 the warmer waters of the Atlantic, Indian and Pacific Oceans. It has only once been recorded from 

 the Mediterranean (Colosi, 1922, p. 13). This species is most frequently captured at or near the surface 

 at night and is only rarely taken in the same waters by day. This suggests a diurnal migratory rhythm — 

 the animals migrating upward at nightfall and descending to greater depths at the approach of dawn. 



The Discovery collection does not extend its known geographical range. 



Siriella gracilis Dana, 1852 



1852 Siriella gracilis Dana, p. 658; 1855, pi. 44, figs, la-g, za-c. 

 1885 a Siriella gracilis, G. O. Sars, p. 209, pi. 36, figs. 25-28. 

 1910 Siriella gracilis, Hansen, p. 31. 



Occurrence : 

 St. 1585. 1. v. 35 (night). Indian Ocean, north-west of Seychelles, 500-0 m., 1 adult <$, 4-4 mm. 



Distribution. This is an oceanic form widely distributed in the warmer waters of the Indian and 

 Pacific Oceans and has not been recorded from the Atlantic. 



Siriella aequiremis Hansen, 1910 



1910 Siriella aequiremis Hansen, p. 40, pi. 3, figs. \a-c; pi. 4, figs. ia-l; 1912, p. 194. 



1919 Siriella aequiremis, Colosi, p. 6; 1920, p. 236, fig. 1 a. 



1937 Siriella aequiremis, Coifmann, p. 3. 



195 1 Siriella aequiremis, W. M. Tattersall, p. 78. 



