60 



G. 0. SAES. 



[No. 7. 



JBairdii But subsequently I was obliged to give up this opinion, 

 having found it to differ very markedly in several points. Thus, 

 the form of the carapace would seem to differ considerably in 

 both, and the peculiar structure of the natatory setae on the 

 antennae, described by King, and likewise found in one of the 

 European species, Alona spinifera Schoedeler, does not exist in 

 the present form. Moreover, the species observed by King would 

 seem to attain a much larger size. In its general habitus the 

 present Alona very much resembles the preceding species, and 

 may at first sight easily be confounded with it. On closer exami- 

 nation, however, several well marked differences are found to occur, 

 e. g. the much larger size of the ocellus, the deviating form and 

 armature of the tail, and finally the absolute want of any sculptur- 

 ing of the shell. 



Description. — The length of the adult female is but little 

 inferior to that of the preceding species, measuring in the larg- 

 est specimens 0.48 mm. 



The carapace, as seen laterally (PI. 6, fig. 5), is of oblongo- 

 quadrangular form, with the posterior extremity somewhat more 

 distinctly truncate than in the last species, the upper angle be- 

 ing well marked, though obtuse. The posterior edges of the 

 valves are somewhat oblique and join the inferior edges by a 

 sharp curve; the latter are nearly straight and as usual densely 

 fringed with delicate bristles. The dorsal edge of the carapace, 

 together with that of the head, forms a continuous curve. The 

 head exhibits a form very similar to that of A. Areheri, though 

 a little broader, perhaps, as seen laterally. 



The surface of the shell, whether in or out of the water, 

 appears quite smooth, without the slightest trace of any sculp- 

 turing, save the usual fine punctation. 



The eye agrees exactly both as to size and structure wi 

 that organ in the preceding species. 



The ocellus, on the other hand, is relatively much larger 

 and but very little smaller than the eye; it exhibits an ob 

 quadrangular form, and is located somewhat nearer to that org* 

 than to the tip of the rostrum. 



