14 G. O. Sara. 



nary manner. Inner duplicatures of valves very large and 

 conspicuous. Head comparatively less thick than in D. 

 Sarsii, and, seen laterally, conically tapering in front, with 

 the end narrowly rounded. Eye comparatively smaller than 

 in the preceding species. Antennæ unusually short, and, 

 when reflexed, scarcely reaching to the end of the inner 

 duplicatures of the valves, terminal joint of upper ramus 

 with only 7 natatory setæ. Caudal part very similar to 

 that in D. Sarsii, outer part of the caudal claws, however, 

 perfectly smooth. Male resembling that of the preceding 

 species; sexual appendages, however, still shorter, scarcely 

 reaching to the base of the caudal claws. Body in both 

 sexes highly pellucid, hyaline. Length of adult female 

 0.76 mm. 



Remarks. — This form is undoubtedly specifically 

 distinct from the preceding one, as also from any of the 

 other known species of the genus, being especially distinguished 

 by the unusually short antennæ, and also by the form of 

 the head (see figs 11, 12). In all the specimens examined, 

 moreover, only 7 natatory setæ were found on the terminal 

 joint of the upper ramus of the antennæ (see fig. 13), 

 whereas their number in the preceding species (see fig. 4) 

 is that normally found, viz. 8. In the full development of 

 the inner duplicatures of the valves (see fig. 14), and in the 

 simple structure of the sexual appendages in the male (see 

 fig. 16), it agrees with the preceding species. Also in their 

 habits no differences are to be found between the 2 species, 

 though perhaps the movements, owing to the less powerfully 

 developed antennæ, are in this form somewhat less vigorous. 



Occurrence. — This form also developed rather 

 plentifully in some of my aquaria; but the mud contained 

 in them was not from the same locality as that from which 



