98 TWELFTH ANIH^UAL REPORT. 



less evident, and all confined to the lower part of tlie shell, while 

 the centre of the valves is marked with very minute striae; the 

 pigment fleck equals the eye; and the post-abdomen is of about the 

 same width throughout and hardly as round below. This and the 

 proceeding species have a well marked keel on the process of the 

 labrura. The size is greater, this being one of the largest and most 

 abundant, as well as one of the most striking species. It, perhaps, 

 should rank as a well marked and permanent variety of the above. 

 Length 0.9 — 1.0 mm. The abdomen of the male is narrowed at 

 the end and lacks the teeth. Lakes about Minneapolis. 



(A small form of A. quadrangularis in lake Calhoun had the eye 

 and pigment fleck equal and the terminal claw smooth.) 



Sp. 10. Alona affinis. Ley dig-. 

 (Plate F. Fig. 14.) 



Lynceus affiinis, LETDIG. 

 Alona affinis, SCHOEDLER. 



Form sub-quadrangular; hight about once and one-half in length; 

 the dorsal outline forming a regular and low curve from end of head 

 to upper posterior marsfin; lower outline very slightly sinuate, 

 anterior one not at all: posterior angles rounded; head nearly hori- 

 zontal; eye of moderate size; pigment fleck considerably smaller; 

 antennules rather large, with unequal sensory hairs at the end, one 

 spine just above the end in front and a bunch of minute hairs near 

 the base behind; antennae comparatively large, basal joint spiny, 

 outer ramus with three setae, two of which have thorns at their 

 middle, also a terminal spine; inner branch with two of the termi- 

 nal setae thorned and the upper lateral seta reduced. The post- 

 abdomen is very broad and short, expanded below and rounded at 

 the end; the terminal claws are straightisb, denticulate, and the 

 spine at the base is also dentate; there is a series of heavy spines 

 on the upper margin of the post-abdomen, accompanied by a series 

 of scales on the side. The shell is unornaraented and fringed below 

 with short bristles. Length 0.9 mm., or more. This fine species 

 is recognized by its smooth shell, the horizontal position of the 

 head, and the form of the post-abdomen; it belongs among the 

 largest of the genus. Lakes near Minneapolis, not rare. 



Birge quotes A. spinifera from Wisconsin. In all probability 

 that species is the younger stage of the above. 



