106 TWELFTH ANNUAL REPORT. 



The form'is rotund, much like species of Chydorus in the highly 

 arched dorsal outline; the beak is rather short and depressed; the- 

 lower outline of the valves is very convex in front, and barely 

 sinuate behind, where it terminates in a minute spine. The shelL 

 is marked, as in no other lynceid, by lines running diagonally 

 backward, and only on the lower part reticulated, if at all. 



The post-abdomen is short, broad and rounded below; the claw 

 has a single basal spine. Length 0.20 mm. — 0.28 mm. This is the- 

 smallest member of the Cladocera. In form it so nearly resembles 

 Chydoras that apon first sight the writer took it for a member of 

 that genus. Our one specimen measured 0.25 mm. The shell is^ 

 marked by plications rather than striae, which arch over the back*. 



Sp. 7. Aloiiella straiat, Schoedler. 



This species is said to resemble A. exigua in habit and sculpture- 

 of shell; the form is quadrangular and not greatly elevated in the- 

 middle; the lower margin is nearly straight and fringed with- 

 bristles; the posterior angle is rounded and unarmed. The anten- 

 nules with their setae extend beyond the beak; the pigment fleck is^ 

 smaller than the eye and half way to the beak. The post-abdomen 

 is long and narrowed toward the end; there are seven or eight anal- 

 spines, and two spines on the terminal claw. Length about 0.5 mm.. 



Sub-genus Pleuroxus. 



Section A. Pleuroxus {verus), Baird. 



This group of lynceids is most obviously defined by the long 

 ''beak", formed by the extension of the chitinous covering of the 

 head. (There is rarely a beak in the sense of that word as applied 

 in the case of Scapholeberis or Daphnia, but the antennules are- 

 simply attached to low Drominences on the under side of a broad 

 shield-like projection of the shell.) This beak-like projection is 

 acute and often long and either curved backward or even bent for- 

 ward. The fornices, or lateral projection of the head-shield, are 

 narrow. The form varies much, but is almost alwaj^s very strongly 

 convex above, and the posterior margin is thus only a fraction of' 

 the whole hight of the animal. In some American species the body 

 is very much elongate, and these also depart from the characteristic- 

 habitus of the s^enus in having strong longitudinal striae instead of 

 reticulations. The lower posterior shell angle has teeth which, in 

 a few cases, extend across the entire posterior margin. The post- 



