54 G. O. Sars. 



species. It may, however, here be noted that, according to 

 Mr. Richard, a form has been recorded under this name by 

 Mi. Wiersejsky from Argentina. 



23. Alona monacantka, n. sp. 



(PI. IX, figs. 5, a-b). 



Specific Characters. — Female: Carapace, 

 seen laterally, of the usual oval quadrangular form, being 

 almost transversely truncated behind, with the upper corner 

 rather slight, dorsal margin evenly arched, ventral nearly 

 straight. Head rather procumbent, dorsal margin evenly 

 arched, rostral projection not very prominent. Surface of 

 shell very distinctly striated longitudinally, from 12 to 14 

 striæ occurring on each side, lower edges of valves finely 

 setiferous, and each forming at the infero-posteal corner a 

 distinct, though small dentiform projection. Ocellus smaller 

 than the eye, and about equidistant between it and the tip 

 of the rostrum. Lip-plate of the usual form, with the lower 

 edge evenly curved. Caudal part comparatively small and of 

 about the same width throughout, supra-anal projection dis- 

 tinct and located about in the middle of the dorsal edge, 

 lower corner obtusely rounded and scarcely projecting at all, 

 denticles of lateral and marginal rows of about the same 

 appearance and rather densely crowded together, caudal claws 

 with an unusually long denticle at the base. Colour whitish, 

 pellucid, with a faint greenish tinge. Length of adult fe- 

 male 0.32 mm. 



Remarks. — This form, at first sight, looks very 

 like the European species, A. rectangula G. O. Sars, both 

 in size and general appearance. On a closer examination, 

 however, it is found to differ in the shape and armature of 



