104 TWELFTH ANNUAL KEPORT. 



haps, for Pleuroxus insculptus.) The various authors who have 

 written of this lynceid have all laid emphasis upon the sculpture of 

 the shell, almost to the exclusion of other points in the description. 

 Prof. Birge has found a quite different form, apparently, which has 

 the same peculiar markings; and even the common Alona oblonga 

 has a part of the valves covered by minute striations. Schoedler's 

 figure of this species is unrecognizable; but, as identified by Kurz, 

 the species seems undoubtedly the same that is common in shallow 

 pools in Minnesota, during autumn, and probably also in Massa- 

 chusetts. 



The variations to which this species is subject are considera- 

 able and may account for the marked disagreement in the 

 accounts of our ditierent authors. Schoedler gives his specimens 

 a length of .20-.25 mm., while Kurz says .35 mm. Birge gives 

 ,27 for the length of Pleuroxus insculptus, and our specimens 

 varied in the same gathering between .24 mm. and .40 mm. 

 Schoedler figures three teeth at the lower posterior angle; Kurz 

 says "several (4);" Birge describes one or two, and Minnesota 

 specimens show gradual transitions from an inconspicuous angle 

 to three or perhaps four teeth. These teeth are the extensions of 

 some of the strong ridges or crenulations which mark the shell. 

 P. E. Mueller's figures of the shell and abdomen of P. exigua 

 would apply to our species perfectly, save the absence of minute 

 striations; Kurz's statements with reference to the differences bet- 

 tween these two forms seem to agree only in part with those of 

 Schoedler. I must here express my suspicion that the Pleuroxus 

 aculeatus, P. exiguus and P. excisus all belong under this species. 

 I have seen a small form which lacked the fine striations; and there 

 appeared to me to be, at times, a slight indication of a second series 

 of hairs upon the post-abdomen. 



The form is oblong, truncate behind, variously arched above, but 

 usually with a rather low, evenly curved dorsal contour; the lower 

 shell margin is either nearly straight or convex in front and con- 

 cave along the posterior third, and is heavily beset with very long 

 pectinate bristles. The head is moderately depressed, with a very 

 broad, blunt and short beak (in some positions this beak seems 

 acute, but it is an optical delusion); the fornices are very broad, 

 covering the antennules completely; seen from above the head is 

 broad and truncate in front; the eye is larger than the large pig- 

 ment fleck, which is nearer it than the end of the beak. The 

 antennse have eight setae, the last of which is minute; the fivr- 



