of the North Atlantic and of North- Western Europe. 631 



Shell of the female (PL lii., figs. 14, 16) narrow and subcylindrical in outline, 

 more than twice as long as high ; ventral and dorsal margins almost straight and 

 parallel throughout the greater part of their length, the arcuation being confined 

 to the extremities, both of which are obtusely rounded ; antennal sinus very 

 narrow, but deep, situated a little below the middle of the anterior margin ; valves 

 highly calcareous, smooth, white. Seen from above the greatest width is behind 

 the middle, whence the sides slightly converge forwards; anterior extremity 

 narrower than the posterior. 



Shell of the male (PI. lii., figs. 12, 13) having the greatest height anterior; 

 ventral margin straight ; dorsal slightly arcuated throughout, and the declination 

 both in front and behind less sudden than in the female ; posterior extremity 

 vertically truncate, with a slight protusion at its junction with dorsal margin. 

 Antennal sinus much more gaping than in female, its lower margin being 

 horizontal instead of projected obliquely forwards, which is the case in the 

 other sex. Seen from above the greatest width is near the front, and the 

 anterior extremity is wider than the posterior. Length of female 2'4 mm. ; 

 height 1 mm. 



In the ? the antennules (PI. l., fig. 3) have the second joint rather shorter 

 than the first, and all the following successively decrease in length ; first joint 

 naked, second with one plumose seta on the inner margin, and one small seta on 

 the side ; third with seven setae on inner margin, of which the five proximal are 

 plumose, the two distal longer, ringed, but not plumose, outer margin with a 

 minute spinule at the base and two ringed setee at the extremity, this joint is 

 subquadrate in form ; fourth joint bearing, at the distal extremity of the outer 

 margin, a sensory organ consisting of a very closely annulated basal portion, 

 which is ultimately flattened out, and then split into six thong-like processes, 

 which are spread on one plane; fifth joint with one small seta on the middle of 

 distal margin, which below carries a conspicuous unguis; last joint with five 

 annulated set£e, from which are thrown out numerous long lashes. 



Antennae (PL l., fig. 5) having the swimming branch bearing eleven setae, of 

 which four are at the extremity, the three most distant smaller than the rest, the 

 last being minute; secondary appendage three- jointed, but apparently imperfectly 

 segmented, last joint terminating in a seta, and another minute seta near the 

 extremity. On PL l., fig. 5, we have represented an abnormal condition of this 

 organ, which on both the antennules of the specimen from which the drawing is 

 taken was of much larger size than usual and entirely without setae. 



The frontal tentacle (PL li., fig. 11) is long and slender. 



The mandible (PL l., fig. 6) is furnished below with a large falcate, reflexed 

 process, the hinder margin of which is armed with four obtuse serrations, and 

 nearer the base a single plumose seta : palp four-jointed, the first joint with three 



4TJ2 



