Fresh-water Entomostraca of South America. 39 



Colour greyish white, upper part of carapace in ephippial 

 females very dark, almost black. Length of adult female 

 scarcely exceeding 0.25 mm. 



Remarks. — This is perhaps the smallest of the 

 Cladocera known, and is hardly more than visible to the 

 naked eye. It is very nearly allied to the European form, 

 S. serricaudatus Fischer (= S. minutus G. O. Sars), but 

 differs in the armature of the tail, and in being of con- 

 siderably smaller size. 



Biological Observations. — While searching 

 for the small Lynceidæ living on the bottom of my aquaria, 

 I several times took up this dwarfed form in the dipping- 

 tube; and on examining with a strong magnifying glass the 

 small quantities thus taken up from the aquaria, and placed 

 in a watch-glass, the specimens were at once perceivable by 

 their slow jumping movements. It is very probable that 

 this form occurred rather plentifully in some of my aquaria; 

 but owing to its extremely small size I only succeeded in 

 securing some few female specimens. In some of these the 

 upper part of the carapace was of a very dark, almost black 

 colour, and such specimens always carried only a single large, 

 opaque ovum in the matrix, most probably a resting egg. 



Occurrence. — The aquaria in which this form 

 was observed, were all prepared with mud from the neigh- 

 bourhood of Sâo Paulo. 



Gen. Ilyocryptus, G. O. Sars. 



.Remarks. — Of this genus, likewise established long 

 ago by the present author, 3 well-defined European species 

 are known, and also 3 exotic forms, viz., I. spinifer Herrick 



