310 Birge — Notes on Cladocera. 



semoling the sketches of the first specimen that the conclusion 

 regarding the distinctness of the species seems warranted and it 

 is described under the above name. 



Female. — The general form is rotund, resembling that of Chy- 

 dorus. The head is large, movable, much depressed. The forni- 

 ces are broad and extend out into a broad, pointed flap of a ros- 

 trum, which can be closely appressed to the valves, The dorsal 

 outline is evenly arched to the posterior margin which is very 

 short, practically absent. The posterior part of the ventral mar- 

 gin rounds over into the dorsal with only a slight break and is 

 fringed with somewhat straggling plumose hairs, longest in 

 front. As seen from below the valves touch about the middle of 

 the ventral edge and are slightly separated at the posterior 

 ^part. They also touch each other anteriorly. In the middle 

 third of the length, the edge of each valve, instead of bending 

 in toward the median line, is turned out, so that a rhomboidal 

 space is here left between the valves. Just posterior to the 

 center of the fold there is a sharp outfolding of the valve, form- 

 ing a groove whose walls are produced ventrally so as to 

 form a sort of curved hollow tooth. In the cavity of the larger 

 fold lies the first foot and in the tooth lies the spine of this 

 foot. • 



The anterior margins of the valves are strongly convex, but 

 not so tumid as is figured in A. emarginatus by Norman and 

 Brady ('67, P. XIX, Fig. 4). This structure of valves and first 

 foot, which is characteristic of the genus, is less fully developed 

 in A. 'minor than in A. emarginatus. In Norman and Brady's 

 figure the outfolding for the spine extends far back toward the 

 posterior edge of the valve, and the spine is at least six times 

 as large as in my specimens, where indeed it was difficult to 

 discover it. The hook of the first foot is said by Sars to be 

 "validus. " Norman and Brady call it a "long, cylindrical fal- 

 cate process, denticulate on the edge, which is very conspic- 

 uous. " In none of my specimens was this true, but the hook is not 

 very large, nor was it conspicuously exserted. 



The antennules are short conical, and bear the usual an- 

 terior sense-hair and cluster of terminal sense-hairs which are 

 about equal in length. The antennae have ff& setae and have no 



