of the North Atlantic and of North- Western Europe. 663 



and bending slightly backwards ; rostrate process in front very broad, its front 

 margin obliquely sloping backwards to the summit, which is obtusely rounded; 

 antennal sinus widely open, its thin marginal laminse almost meeting each other ; 

 the sinus looks downwards, its lower margin is not convex, and sloping back- 

 wards unites at an angle with the ventral margin. Surface of valves very rugose, 

 and boldly sculptured with elevated ribs. The lowest of these commences at the 

 bottom of the antennal sinus, and then passes round it, and round the ventral 

 margin, a little within its edge, to the posterior extremity. Midway between the 

 bottom of the sinus and the point where this rib turns to pass round the ventral 

 margin, it throws out a branch rib, which traverses the valves a little below the 

 middle, until it nearly reaches the posterior margin ; a third rib, commencing at 

 the extremity of the rostrate process, sweeps across the valves until it comes near 

 the summit of the hinder margin, there it is deflexed, and proceeds parallel with 

 the hinder margin, and finally unites with the first described rib at the infero- 

 posteal corner ; a fourth and fifth rib also commence in the rostrate process, and 

 pass backwards, the one near to, and the other on the back of the valves, at 

 about half the length of which they coalesce, and proceed backwards near the 

 dorsal margin as one rib (PI. LVi., fig. 12). These last ribs are best seen when 

 the shells are looked down upon from above. The entii-e surface of the valves 

 between the ribs is pitted with foveolse. Seen from above the form is sub- 

 hexagonal, with nearly jjarallel sides, which converge suddenly and angularly 

 towards the extremities, the anterior of which is widely and abruptly truncate, 

 the posterior broadly and bluntly mucronate and notched in the middle. The 

 valves are very tumid, so that the breadth is more than half the length. The 

 end view is irregularly heptagonal, the nearly parallel lateral margins ending 

 above and below in prominent rounded angles, the two superior margins con- 

 verging into an irregular arch ; the basal margin nearly flat. Length 2 '6 mm. ; 

 height l'7mm. 



The antennules end in six long and some short setae. The smaller branch of 

 the antennse has six setae at the base, on the middle a great plumose, annulated, 

 sensory seta (PL lvi., fig. 10), and a minute curved spine-like seta at the 

 extremity ; the swimming branch bears fourteen setae. The frontal tentacle is 

 of great length, and slender. The mandible has the masticating lobe as usual, 

 bifid and hispid, and the masticating spines have one strong lateral prong, with 

 more slender spine-points beyond it. The second maxillae are indistinctly four- 

 lobed, the first three lobes beset densely with setae, fourth lobe having the outer 

 tooth-process very large and strong, often with a lobe above near the base ; inner 

 edge plain, that is, not denticulated, the succeeding and underlying series of teeth 

 gradually diminishing in size, all with their inner edge denticulated, and 

 some setae springing from base of the large tooth ; beyond is the palp, which is 



6 A2 



