120 THE VOYAGE OF H.M.S. CHALLENGER. 



liristlcs, t(M-niiiial joint very small and spinous. Eemaining pairs of legs successively 

 diminishing in size. Penultimate pair of legs exceedingly small, endopod bi-articulate, 

 exopod distinct. Last pair of legs quite rudimentary, forming only a small liuguiform 

 and setose plate. The three posterior pairs of gills sending off a branch inwards ; exterior 

 branch in all as in Nyctiphanes. Luminous apparatus of the usual structure. 



Remarks. — This genus was established by Brandt for the reception of an Arctic species 

 from the Siberian Sea, Thysanoessa loiKjipes. I have recorded' two species from the 

 Norwegian coast, Thysanoessa horealis and Thymnoessa tenera, and in the Challenger 

 collection there are two other species described below. Thus the genus comprises at 

 present five different species. They all agree in the characters stated above, the most 

 striking of which is the strong development of the first pair of legs. 



The two Challenger species may be briefly characterised in the following manner : — 



Loily 



comparatively clumsy, with last segment only slightly longer than preceding. 

 Rostrum narrow, lanceolate. First pair of legs exceeding half the length of 

 body. Eyes remarkably large, . . . . . . T. gm/an'a, n. sp. 



very slender, with last segment exceedingly elongate and narrow. Rostrum 

 triangidar, broad at base. First pair of legs not nearly attaining half tlie 

 length of body. Eyes of moderate size, . . . . . T. macruni, n. sp. 



31. Thysanoessa gregaria, G. 0. Sars (PL XXL figs. 8-17 ; PI. XXIL). 



Tlujsanoessa gregaria, G. 0. Sars, Preliminary Notices on the Challenger Schizopoda, No. 29. 



Specific Characters. — Form of body rather short and clumsy. Carapace with a well- 

 marked lateral denticle behind the middle of the inferior margin ; rostrum produced, 

 straight, narrow lanceolate. Caudal segments smooth above, with slightly angular epimera. 

 Last segment somewhat longer than preceding, preanal spine very large, forming a broad 

 plate, serrate at posterior edge. Eyes remarkal^ly large, irregularly globose, upper part 

 of cornea narrowed. Antenna! scale reaching beyond second joint of anteunular peduncle, 

 oblong, tapering, apex obliquely rounded, with outer corner projecting. First pair of 

 legs, when extended, exceeding half the length of body, meral joint reaching tip of 

 antennular peduncle. Telson with two pairs of dorsal denticles, apex acuminate ; sub- 

 apical spines smooth. Inner plate of uropoda a trifle longer than outer, scarcely 

 reaching tip of telson. Length 1 8 mm. 



Rema7']cs. — This species may at once be distinguished from the three northern forms 

 by its relatively short and clumsy body, as also its remarkabl)^ large eyes. Moreover, 

 the peculiar form and strong development of the preanal spine would seem to afibrd a 

 good specifit; character. 



Description. — The average length of adult females is about 18 mm. The males are, 

 as a rule, somewhat smaller. 



1 Oversigt over Norges Crustaceer, Bd. i., pp. 52, 5.3, 1882. 



