260 GEOL. AND NAT. HIST. SURVEY OF MINNESOTA. 
P. glaber Schoedler is P. personatus. 
P. brevirostris Schoedler is P. truncatus. 
P. aculeatus Fischer is Alonella exigua. 
P. convexus Poggenpol is doubtful. 
SECTION B. Leptorhynchus Herrick.!—The species for which Sars 
formed the genus Harporhynchus is of Alona-like habit, but has a beak 
which ‘exceeds that of any known Pleuroxus in length, being sim- 
ulated in this respect by the American P. acutirostris, which is, how- 
ever, in other respects more nearly allied to Plewroxus. 
Leptorbynchus faleatus Sars. 
Harporhynchus falcatus —Sars. 
Alona falcata—Sars, P. E. Mueller. 
Body oblong, arched above; ventral margin nearly straight, setose, 
with a spine at the posterior angle; beak strongly curved, folded 
laterally; pigment spot larger than the eye. The post-abdomen is 
wide, sides nearly parallel, armed with a few strong teeth below and 
a lateral line of spines; caudal claw with a single small basal spine. 
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. Occa- 
sionally they may be seen in sunshiny weather, disporting 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 probably is not confined to any periods. 
The males, which only rarely are found even in these periods, have 
the abdomen narrowed or excavated to accommodate it to the peculiar 
alteration of the brood cavity which takes place in the sexually ma- 
ture female. The connection takes place by the insertion of the abdo- 
men 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 
LXIV, 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. 
1 Harporhynochus is preoccupied in ornithology. 
