120 TWELFTH ANNUAL REPORT. 



cryptus, this animal passes its life in filth at the bottom of pools, 

 and rarely emerges to the light of day. What little visual function 

 there may be is vested in the larval organ. 



The specimen from which the drawing was made measured 0.45 

 mm. The first glance at this rarest of all entomostraca afi'ords proof 

 of its unique character. The strongly arched shell is so compressed 

 as to bear little resemblance to Chydorus. The dorsal line passes 

 with little angle into the high posterior margin. There is a rounded 

 angle below, armed with two teeth — the shortened representatives 

 of the fringing spines of the straight lower margin. The head is 

 depressed and very short, but the narrow beak is produced to below 

 the margin of the valves. It is rounded so as to resemble, as seen 

 in front, a duck's bill. The fornices are narrow and flare so that 

 the eye is left partly exposed upon the side. The antennules are 

 not long but slender. The labium has a very large lamella, which 

 is crenulate in front and acute below, the labrum proper being large. 

 The systematic position of this genus is a matter of considerable 

 interest, for it is the only member of the whole order in which the 

 larval eye is the only one developed, and the first thought would 

 be that this must be a primitive synthetic type, in other words, 

 historically the oldest of Cladocera. Closer study does not warrant 

 the theory. There is much to indicate that, though essentially lyn- 

 ceid, it stands in close connection with the higher members of the 

 familj' and perhaps has more than a superficial resemblance to such 

 degraded lyncodaphnids as Ilyocryptus. All things considered, 

 however, our diagram stands with this genus as a degraded offshoot 

 of the more typical stem of Lynceidae. 



SUB-ORDER II.— GYMNOMERA. 



This group is easily recognized by the almost entire absence of 

 the shell, which forms so conspicuous a part in the greater number 

 of the Cladocera. Here it serves simply to form a pouch or brood- 

 sac for carrying the eggs and embryos. The feet are nearly terete 

 and prehensile, with but slight indications of branchial appendages. 



FAMILY POLYPHEMID^. 

 Feet five pairs. Antennae with the rami three- or four-jointed. 



I. — Genus Polyphemus, De Geer. 

 Head very large, separated by a depression from the compact 



