STATE GEOLOGIST. 115 



Sp. 1. Plirixura rectirostris, P.E.Mueller. 



(Plate I. Fig. 18.) 



Beak acute; shell striated longitudinally, slightly arched above; 

 ventral margin rounded, with a round and unspined angle behind. 

 Length 0.5 mm. Not yet encountered in Minnesota. 



IX. — Genus Chydorus, Leach. 



This genus, if it be really of generic value, contains minute 

 rotund animals which appear in the water like animate pin-heads 

 of small size. Their motion is a rolling, wavering hobble; and they 

 live by preference upon vegetation, or in slime at the bottom of 

 pools. Occasionally they may be seen in sunshiny weather, dis- 

 porting themselves near the surface in immense numbers. There 

 are two common species, and six more which are more rare or in 

 part not valid. 



The sexual period occurs at two different periods (i. e., May — 

 June, and December), but in probability is not confined to an}'' 

 periods. The males, which only rarely are found even in these 

 periods, have the abdomen narrowed or excavated to accomodate it 

 to the peculiar alteration of the brood-cavity which takes place in 

 the sexually mature female. The connection takes place by the 

 insertion of the abdomen within this chamber, which is facilitated 

 by the reduced size of the abdomen. The modification of the shell 

 of the brood-cavity above referred to consists in the thickening of 

 the wall posteriorly, which may or may not result in the deforming 

 of the shell, as shown in plate F, fig. 7, taken from Kurz. This 

 may be termed an ephippium, although it differs somewhat from 

 the modified shell so called in Daphnia. The male element consists 

 of nearly round nucleated cells, and the opening of the vas deferens 

 is anterior to the terminal claws. The members of this genus are 

 among the most minute forms of the family or the entire group. 

 Concisely put, the characters are as follows: 



Form globose, not obviously truncate behind; head terminating 

 in a sharp, long, curved beak, which lies close upon the anterior 

 margins of the valves; antennas short; eye larger than the pigment 

 fleck; abdomen flattened, excavated in the male; intestine with no 

 anterior cseca, doubly convoluted, with an anal csesum. Three spe- 

 cies found in Minnesota. 



