202 

 99. Cytherura nigrescens, (Baird). 



(PI. XCIV, fig. 1). 

 Cy there nigrescens, Baird, British Entomostraca, p. 171, PI. XXI, figs. 4, 4 a. 



Specific Characters. Female. Shell rather compressed, seen laterally, 

 oval in shape, about equally high in front and behind, greatest height slightly 

 exceeding half the length, dorsal margin gently arched, joining the anterior 

 and posterior edges without any intervening angle, but sloping somewhat more 

 steeply behind than in front, ventral margin slightly sinuated in the middle, 

 anterior extremity rounded off, posterior exerted to a well defined conical 

 protuberance somewhat obliquely truncated at the tip and located about in 

 the longitudinal axis of the shell; seen dorsally, narrow oblong in outline, 

 with the greatest width somewhat behind the middle and about equalling 2 A> 

 of the length, 1 side-edges rather evenly curved, both extremities pointed, the 

 anterior somewhat narrower than the posterior. Valves slightly unequal and 

 rather thin, with the surface quite smooth, without any obvious sculpture, 

 being only marked by scattered small knobs; inner duplicatures remarkably 

 broad, that in front limited inwards by a somewhat irregularly waved line 

 sending off to the edge a number of very conspicuous fine pore-channels 

 partly arranged in bundles, that of the hind extremity strongly bowed inwards; 

 edges smooth and clothed with scattered very fine hairs. Anterior antennae 

 very slender, with the terminal joint rather small, not nearly attaining half th 

 length of the preceding joint. Posterior antennae with the dividing suture 

 the penultimate joint located considerably above the middle. Legs com 

 paratively small. 



Immature specimens conspicuously differing from the adults in the shape 

 of the shell, which gradually tapers behind to an obtuse point, the dorsal 

 margin being boldly and evenly arched, whereas the ventral one is nearly straight. 



Male very like the female, but of smaller size, and having the shell, seen 

 laterally, somewhat narrower, with the dorsal margin nearly straight. Copu- 

 lative appendages rather unlike those of the preceding species, basal part 

 obliquely oval in shape and without any processes below, copulatory string 

 comparatively short, bent in a circle, with the extremity deflexed; terminal 

 part rather small, securiform in shape. 



Colour of shell in the greater extent very dark, almost black, extremities 

 however much lighter and semipellucid. 



Length of adult female scarcely exceeding 0.43 mm. 



Remarks. This form also has been recorded at a rather early date, viz. 

 in the year 1850, being described by Baird as a species of the genus Cythere 



lae 



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