CHIEFLY IN NORTHUMBERLAND AND DURHAM. 869 



numbers, except in a dredging recently made by Mr. Robert- 

 fr<on in Loch Long, where it occurs abundantly. 



Genus. XIPHICHILUS,* nov. gen. 



Shell thin and fragile, smooth; valves compressed, elongated, 

 pointed at both ends, nearly equal, ventral margins much com- 

 pi'essed, forming a flattened knife-like plate which is widest be- 

 hind the middle, and marked by several transverse, hair-like 

 lines : outline, as seen from above, compressed, bi-fusiform ; 

 hinge simple. Limbs excessively long and slender : superior 

 antenna six-jointed, and quite destitute of setas, inferior spar- 

 ingly setose; mandibles very long and slender, styliform, palp (?) 

 bi-articulate, slender, and terminating in two long set£e. Abdo- 

 men produced into two long tapering processes. 



Only two members of this genus are yet known, both of which 

 are here for the first time figured. Though very nearly allied, 

 in internal anatomical structure, to Paradoxostoma, the charac- 

 ters of the shell are such as to suggest the propriety of placing 

 them in a distinct genus, and some slight anatomical differences 

 appear to confirm this view. The very remarkable external ap- 

 pearance of the shell will be at once recognized on reference to 

 the plates. 



XiPHiCHiLus TENUissiMA, [Norman.) Plate XII., figs. 6-9; and 

 Plate XIV., figs. 5-10. 



BytJiocy there tenuissima, Norman. Brit. Assoc. Report 

 (1868), p. 294. 



The shell of this species has been well described by Mr. Nor- 

 man, and was by him referred (owing to some general likeness 

 to that of B. simplex) to the genus Bythocythere. But an ex- 

 amination of the contained animal, which I have been fortunate 

 enough to obtain recently in good condition, compels me to re- 

 move it from the position provisionally assigned to it by Mr. 

 Norman. 



K'l'PoQ a sword ; xuKq^ a margin. 



