these true natatory setae are about of uniform length and exhibit 

 a very short and indistinctly defined basal joint. 



As to the structure of the several limbs, it agrees on the 

 whole so closely with that found in other species of the genus, 

 that I do not find it necessary to give any complete description, 

 but only to speak of those parts, in which the specific differences 

 generally are more obvious, viz., the anterior antennae and the 

 last pair of legs. 



The anterior antennae (see figs 12 and 13), as usual, form 

 each a very elongate and slender stem, tapering to the extremity 

 and consisting of 25 distinctly defined joints bearing short setae 

 especially along the anterior edge. During the movements 

 of the animal, these antennae are borne expanded to either side, 

 nearly at right angles with the axis of the body. In the present 

 species they exhibit however near the base a rather sharp curve, 

 whence their outer straight part points somewhat obliquely 

 posteriorly (see fig. 12). When reflexed along the sides of the 

 body (see fig. 13), their tips reach a little beyond the 1st caudal 

 segment. Thus their length somewhat exceeds that of the same 

 organs in the European form, B. castor (Jurine), whereas they 

 are shorter than in the other species mentioned above, D. dm- 

 ticornis Wierzejski, in which they reach the length of the whole 

 body. 



The last pair of legs (fig. 14) exhibit, as in the other 

 species of the genus, an aspect very different from that of the 

 4 preceding pairs, which constitute the chief natatory organs 

 of the animal. Notwithstanding this dissimilarity, it is not 

 difficult to see, that they are composed of the same principal 

 parts, though very much modified in structure. The basal part 

 consists of 2 very distinctly defined and movably connected seg- 

 ments, generally forming with each other a more or less pro- 

 nounced angle. The 1st of these segments is the larger and 

 nearly globular in form; the 2nd is somewhat compressed and 

 slightly dilated at the end, bearing at the outer corner a small 

 bristle. To the extremity of the latter segment the 2 rami are 

 attached, the outer of which is by far the larger and, a 



