30 



This and the following species will be more briefly described. 

 There is no evident angle at the junction of the dorsal and pos- 

 terior margins. The lower edge bears setas and has no sinns. 

 The front edge has a very slight sinus, or none at all. The 

 valves are marked by faint horizontal striae. The rostrum does 

 not extend so far downwards as in the preceding species. The 

 post-abdomen has its usual claws, each with its basal spine, which 

 is not serrate. The teeth of the post-abdomen are about twelve 

 in each row ; three or four at the end are larger than the rest. 

 There is besides, a row of hairs above the row of teeth. The 

 lower angle of the post-abdomen is not rounded. The male is of 

 the same general shape as the female. Length, 0.34, height, 

 0.18 mm. In the armature of the post-abdomen this species ap- 

 proaches nearest to A. tenuicaudis (Sars) ; but in other respects, 

 e. g. shape of shell, and especially of post-abdomen, is quite dif- 

 ferent. 



Cambridge, Mass., 1876.; Madison, Wis., July, 1877. Not com- 

 mon. 



SPECIES 3. 



ALONA GrLACIALIS. sp. UOV. 

 Length, 0.3 mm. Height, 0.19 mm. 



Anterior margin of valves without sinus. Yalves horizontally 

 striated. Lower corner of post-abdomen rounded. 



This species differs from the preceding chiefly in the post-ab- 

 domen. This is rounded at the lower angle, and the teeth, about 

 fourteen in number, are of equal size. There is also a second row 

 of hairs. The forward edge of valve is strongly convex, other- 

 wise much like A. porrecta. This species approaches perhaps most 

 nearly to A. lineata (Fischer). There are, however, great dif- 

 ferences. The size of A. lineata is nearly twice as great. The 

 post-abdomen has, according to Miiller and Schodler, only one 

 row of teeth and no hairs. According to Kurz, it has hairs, but a 

 deep incision in the lower end. In either case the dtfference is 

 well marked. The shape is also different. A. glacialis is rela- 

 tively much broader behind than A. lineata (vid. Schodler). 



