NO. 1848. RECENT CLADOCERA FROM NEW ENGLAND— DOOLITTLE. 165 



specimens the end of the postabdomen has withdrawn or shrunken 

 from the exoskeletal sheath, apparently showing the opening of the 

 sperm duct, developing for a molt soon to occur, terminal and dorsal 

 to the caudal claw. 



Color light yellow; test and tissues transparent. 



Measurements of an egg-bearing specimen taken from Anonymous 

 Pond (Crystal Lake), Maine, United States of America, are: 



mm. 



Length 1.14 



Maximum lieight 82 



Posterior height 12 



Meshes, diameter 021- 018 



Eye, diameter 0137 



mm. 



Antennules, length 0. 20 



Antennules, diameter 02 



Postabdomen, length 24 



Terminal claw, length 12 



Ocellus, diameter 0112 



Type. —Cat. No. 42781, U.S.N.M. 



Type-locality. — Anonymous Pond (Crystal Lake), Maine, Septem- 

 ber 5, 1908. 



Occurrence. — Taken frequently and singly in weedy shallow parts 

 of Umbagog Lake, Maine and New Hampshire, August, 1905; Sebago 

 Lake, Maine, July, August, September, 1906, 1907, 1908; Anony- 

 mous Pond, Maine, September, 1908. 



Family CHYDORID.E Leach (LYNCEID/E Baird). 



CHYDORUS BICORNUTUS Doolittle. 



Plates 17, 18, and 19. 



Chydorus hicornutus Doolittle, Proc. Biol. Soc. Washington, vol. 22, 1909, 

 p. 154. 



Female. — Length of body 0.50 to 0.62 mm.; height of body five- 

 sixths the length. General form from side view circular oval, some- 

 what truncated posteriorly, posterior margin sparsely spined. Ven- 

 tral margin sinuate, with sharp ventral curvature at the middle; 

 setigerous margin turned inward, with short, delicate ciliated setae 

 interlocking across the ventral opening or gape between the valves. 

 Seen from above, body nearly twice as long as broad, oval, broadest 

 at middle, rounded broadly anteriorly, tapering posteriorly. In 

 front view general form oval, broadest at upper one-fourth, dorsal 

 margin slightly concave on each side. The ventral margins of the 

 valves nearly approximate in posterior one- third, but slightly diver- 

 gent at posterior one-third and running forward nearly parallel. 



The form of the test of the body proper is modified by a most 

 extraordinary exoskeletal development of horns, ridges, and deep 

 rectangular and hexagonal cells, of which the exoskeletal develop- 

 ment of C.faviformis Birge gives a suggestion. From the middle of 

 each valve of the test there stands out horizontally a large horn, 

 slightly curved posteriorly. This horn varies in length from one-fifth 

 of to a little more than the width of the body. From this obvious 



