106 TWELFTH ANNUAL REPORT. 



The form is rotuud, much like species of Chydorus in the highly- 

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

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

 sinuate behind, where it terminates iu 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 

 Chydorus that upon 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 stri^, which arch over the back. 



Sp. 7. Alonella 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 setse extend beyond the beak; the pigment fleck is 

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

 is long aud 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 autennules^ are 

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

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

 acute aud 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 always 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 genus 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— 



