101 



ovate in outline, greatest width behind the middle and about equal to 2 ,3 of 

 the length, anterior extremity sharply pointed, posterior obtuse. Ventral im- 

 pressed area of a somewhat navicular shape and occupying the greater length 

 of the shell, being defined on each side by a well marked keel. Surface of 

 valves smooth, with scattered delicate hairs at each extremity, infero-posteal 

 corner of left valve produced to a short tooth-like prominence; both valves 

 exhibiting in front a rather broad pellucid marginal zone. Anterior antennae 

 with the basal part rather dilated, terminal part slightly longer and having the 

 1st joint much larger than the others, almost occupying half the length of 

 that part. Posterior antennae with the last joint fully as long as the penulti- 

 mate one and carrying on the tip a slender setiform claw accompanied by 2 

 or 3 small bristles, claws of the penultimate joint replaced by 3 setae one of 

 which is peculiarly bent at the tip; natatory setae of these antennae extending 

 as far as the apical claw. Caudal rami of moderate size and only slightly 

 curved, apical claws gradually diminishing in length proximally, the distal one 

 somewhat exceeding half the length of the ramus. 



Male of somewhat larger size than female, and conspicuously differing 

 from it in the shape of the shell, which, seen laterally, appears conspicuously 

 dilated behind the middle, with the ventral margin in that plase considerably 

 bowed, the posterior extremity exerted to an obtuse lappet and wanting any 

 trace of a dentiform projection. Prehensile palps of maxillipeds very power- 

 fully developed and conspicuously dissimilar both in size and shape; propodos 

 of left palp somewhat lamellar, sub-oval in form, with the inner edge angularly 

 bent near the middle and the outer edge terminating in a somewhat securi- 

 form lamella; propodos of right palp almost twice as long and narrow fusi- 

 form in shape, with a small seta at the end inside; dactylus of both palps 

 slender, somewhat compressed at the base and exerted to a thin setiform lash. 

 Caudal rami considerably more curved than in female and also compara- 

 tively narrower. 



Colour in both sexes rather peculiar: upper part of the shell of a pale, 

 opaque whitish hue gradually passing over below to a dark, almost black 

 shadow, which extends along the lower part of the shell and curves upwards 

 in front, so as to occupy the greater part of the anterior extremity. 



Length of adult female amounting to 1.10 mm., of male to 1.20 mm. 



Remarks. This easily recognisable Ostracod was described as early as 

 the year 1785 by O. Fr. Miiller, and has subsequently been observed by many 

 other authors, all of them having adopted for it the specific name originally 

 proposed, with the only exception of Koch, who did not recognise this form, 



