10 University of California Puhlications i)i Zoology [Vol. 13 



Subfamily Nematoscelinae Holt and Tattersall 



''Eyes more or less bilobate. Second or third legs elongate, with 

 distal extremity forming a brush or claw" (Holt and Tattersall, 1905, 

 p. 107). Caiman (1905, p. 154) gives a key for the genera of this 

 family. 



Genus Thysanoessa Brandt 



Thysanoessa, Sars (188-5). p. 119. 

 Thysanoessa, Holmes (1900), p. 229. 

 Thysanoessa, Zimmer (1909), p. IS. 

 Thysanoessa, Hansen (1911), p. 36_ 



Hansen (1911, p. 36) considers that the usual diagnosis of this 

 genus is insufficient and he gives a new definition of it, including with 

 the forms previously known as Thysanoessa those called Rliocla Sim. 

 The rostrum is always of good size, and the eyes, while generally con- 

 stricted into a narrow upper and a broad lower portion, may be almost 

 circular. The two distal joints of the antennules are more slender in 

 the females, and the flagella are short in both sexes. The first six 

 pairs of legs are always normally developed and the second pair may 

 be much lengthened and thickened; if so, the last two joints have 

 strong spine-like bristles along both edges (pi. 2, see fig. 31, and Zim- 

 mer, 1909, p. 21, fig. 32). The last two pairs of legs are reduced and 

 modified and show sexual differences. Hansen states that in the 

 seventh (sixth, according to his usage) pair the outer ramus is nor- 

 mally developed and the inner ramus wanting in males, while in 

 females the exopod is always present, but may be either unjointed 

 or 2-jointed. The last pair is without the inner ramus. 



Thysanoessa is allied to Nematoscelis, but readily separable from 

 it because of the structure of the second pair of legs, for in the latter 

 genus the two terminal joints of the elongate legs do not carry bristles 

 along the edges. 



Key to the Species of the Genus Thysanoessa 



1. Third or fourth segment of the abdomen with a dorsal process extending 

 from the posterior margin spinifera 



1. Third and fourth segments of abdomen without a dorsal process 2 



2. Eyes bilobate; denticle on margin of carapace placed anterior to the middle 



line raschii 



2. Eyes nearly circular (not constricted), denticle on margin of carapace located 

 well behind the middle line gregaria 



