112 TWELFTH ANNUAL REPORT. 



or nearly straight, covered by long pectinate bristles. The head is- 

 short, and the beak is long and sharp; the antennules are of moder- 

 ate size, with a lateral seta one-fourth from the end; pigment fleck 

 less than the eye; antennae rather long, with strong thorns on the- 

 terminal joints. The post-abdomen is long, as in P. hastatus, sides 

 nearly parallel; anal teeth sharp, small and numerous; claws pec- 

 tinate, with two strong basal spines. The shell is strongly striate- 

 with luDgitudinal strise, which are parallel with the different mar- 

 gins. Birge says that there is a reticulated area. The lower angle 

 is rounded, and anterior to it is a small tooth directed backward. 

 This species is distributed throughout the Mississippi valley. I 

 have notes of it from Swan lake, near Decatur, Alabama. It i& 

 often rather abundant about Minneapolis, but is thought by Birge 

 to be absent from the eastern states. Almost all the specimens I 

 have seen are very dark, often brown, so as to appear to the eye 

 like dark specks as they swim about. The length varies froui 0.55 

 mm. to 0.85 mm. About 0.60 mm. is a common size, according to 

 my observation. Birge mentions a rudimentary sixth foot in this 

 species. This organ is found in Eurycercus and other lynceids» 

 according to Schoedler. 



Sp. 11. Pleiiroxus trimcata, 0. F. Mueller, 



LynceiLS truncatus, mueller, koch, zaddach, lievin, fischer, liiLLjEBORG, let- 

 dig, FRIC. 

 Percantlia truncata, baird, schoedler, kurz. 

 Pleuroxus truncatus, p. e. mueller. 

 Percantha brevirostris, schoedler. 



This species is widely distributed in Europe, but is replaced in 

 America by the following. The shell is high, the dorsal contour 

 arched; beak rather long and straight; lower margin slightly con- 

 vex, setose; posterior margin straight, armed with very strong teeth 

 entirely across it; the anterior margin also is dentate, as in the next. 

 The valves are covered with strong strise, springing from the an- 

 terio-central part and radiating toward the free margins. The post- 

 abdomen is of moderate size and in form much as in the next. The 

 ephippium causes a considerably change in form and coloration. In 

 the male the beak is shorter, and the abdomen has finer teeth. The 

 first foot has an extraordinarily large hook. The length is about 

 0.5 mm. to 0.6 mm. Percantha brevirostris,Schoedler, differs in the 

 length of the beak only. 



