Genus THYSANOESSA, G. O. Sais 



Thysanoëssa parva, H. J. H. 



Of this species I have now looked through more than 1600 

 spécimens taken in 14 stations during 1904; ail thèse stations 

 are situated in the triangular area : Banc de Gorringe, the 

 Azores, the Canary Islands. 



Nearly ail adult spécimens measure only o,-io mm , but some 

 few spécimens are larger. The largest maie seen measures 1 1 ,5 mm , 

 the largest female i3 mm ; both thèse spécimens are from stat. 

 1794. 



Among the différences between T. parva and T. gregaria 

 pointed out in my former paper those drawn from the setœ on 

 the elongate (first) pairs of legs are the best. Here I can add 

 another still better character. In T. gregaria the endopods of 

 second, third and fourth pairs of legs have the setse along the 

 lower margin of fifth and sixth joints robust and long, as long 

 as, or longer than, the terminal setœ on seventh joint, and 

 strongly « ciliated », viz. adorned with numerous short latéral 

 hairs easily seen with an enlargement of about 3o times (com- 

 pare figs. 11, 1 3 and 14 on Pl. xxn in the Challenger Report). 

 In T. parva the setas along the lower side of the same joints of 

 second-fourth legs are thin, much shorter than some of the 

 terminal setœ, and even with an enlargement of more than 

 100 times no vestige of « ciliae » can be discovered. Rather 

 si mil a r différences are found between the setœ on the lower 

 side of fifth and sixth joints of the maxillipeds, but in thèse 

 appendages one of the terminal setœ is exceedingly long and 

 ciliated in both species. Therefore, if only one single endopod 

 of one of the four anterior pairs of legs, or even only a maxil- 

 liped, has been preserved it is easy to décide whether the spé- 

 cimen in question belongs to T. parva or it is a small spécimen 

 of T. gregaria. If a spécimen measuring 10 to i3 mm has lost 

 ail the five pairs of appendages mentioned, it may sometimes 

 be difficuit to détermine it with absolute certainty. 



(42) 



