398 MRS. E. W. SEXTON ON A [Apr. 28, 



imuiature, and lightly excavated, almost entire, in the adult ; in 

 Mr. Stebbing's species, " perhaps not fully adult," deej^ly incised. 

 The incision appears to be a chai-acter of the j^oung, but this point 

 cannot be settled till the young of T. vicceense and the adult of 

 the last species ai-e known. 



This genus is a deep-water form, not being recorded from less 

 than 60 fathoms in northern seas, 30-40 fathoms in southern 

 waters. 



IV. Definition of Genus. 



Family Lysiaiiassidae. 



Genus Trischizostoma Boeck & Esmark, 1860. 



1853. Cruerinia (preoccupied, 1830) A. Costa (1). 



1860. Trischizostoma Boeck (3). 



1865. „ Lilljeborg (5). 



1870. „ Boeck (7). 



1872. „ Boeck (8). 



1886. „ Bovallius (11). 



1888. „ Stebbing (12). 



1890. „ G. 0. Sars (13). 



1893. Trischizostoma & Guerina Delia Yalle (15). 



1895. Trischizostoma Stebbing (,6). 



1905. GtterineMa Chevreux (18). 



1906. Trischizostoma Stebbing (19). 



Body thick, slightly compressed, rounded dorsally. 



Head longer than first segment of perpeou, with a short, 

 anteriorly-rounded I'ostrum, produced over the bases of the 

 superior antenna?. 



Uyes very large, contiguous dorsally, occupying the whole sur- 

 face of the head, except the rostrum and infei'O-lateral margin. 



/Superior antenna : peduncle very stout, with fii'st joint larger 

 than the other two ; first joint of piimaiy flagellum so large as to 

 appear a continuation of the peduncle, thickly fringed with sensory 

 filaments ; accessory flagellum 3- or 4-articulate, with the first 

 joint long and laminar. 



Inferior antenna much longer than supeiior antenna ; flagellum 

 in male longer than in female. 



Oral parts greatly projecting inferiorly. 



Mandibles with large body carrying very large 3-articulate, 

 densely setose palp ; cutting- edge truncate, unarmed ; no molar. 



Maxilla 1 ; outer plate elongate, slightly contracted just below 

 the apex, which is divided into 5 claw-like teeth ; inner plate 

 small, unarmed ; palp small, bi-articulate. 



Maxillipeds laige, outer plates partly encompassing the oral 

 parts ; innei' plates nari'ow ; palp 4-articulate. 



Perceon : last two segments smaller than preceding ; infero- 

 posterior angles of segments produced backwards and rounded. 



Side-plates : 1st small, nearly triangular, almost completely 



