>1818.] OF THE UNITED STATES. 427 



SPECIES. 



1. A. communis^. Body oblong oval, furnished with 

 short rigid hairs;, interior antennce equal to the peduncle 

 of the exterior ones; caudul appendices^ peduncle de- 

 pressed. 



Inhabits small streams of fresh water, under stones. 



Cabinet of the Academy. 



Body oblong-oval, a little narrowed before, segments 

 transverse, subequal, indistinctly emarginate on the edges 

 each side, hairy; hairs very short on the disk, longer on 

 the edges and feet; third and fourth segments linear, the 

 anterior ones a little curved forwards and the posterior 

 ones backwards; head naiTower than the first segment, 

 and not longer; superior antenna extending to the base of 

 the tail; inferior antenna equal to the peduncle of the su- 

 perior ones; eyes obovate, oblique, prominent, black; tail 

 as broad as the segments of the body, transverse-subor- 

 bicular, equal to the two last segments conjunctly, depres- 

 sed, and a little prominent between the appendices; ap- 

 pendices as long as the tail, laciniae subequal, peduncle 

 dilated; anterior feet hardly longer, monodactyle, unarm- 

 ed; thumb as long as the hand; hand oval; carpus triangu- 

 lar; remaining feet gradually longer to the hind pair^ which 

 is longest, first and second joints suboval, gibbous above, 

 third joint triangular, extended over the base of the suc- 

 ceeding one and tipped with long hairs, fourth and fifth 

 linear; tarsi half as long as the preceding joint, simple, 

 acute. 



Length one fourth of an inch^ breadth less than one 

 tenth. 



A very common species in our fresh waters, pai'ticular- 



