258 GEOL. AND NAT. HIST. SURVEY OF MINNESOTA. 



Taries from 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. 



Pleuroxiis tnincata O. F. Miiller. 



Lynceus truncatus — Miiller, Koch, Zaddach, Lievin, Fischer, Lilljeborg, Leydig, Fric. 

 Pereantha truneata — Baird, Schoedler, Kurz. 

 Tleuroxus truncatus — P. E. Mueller. 

 Percnntha 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 convex, 

 setose; posterior margin straight, armed with very strong teeth en- 

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

 The valves are covered with strong striae, springing from the anterio- 

 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 considerable 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. 

 Percantlia brevirostris Schoedler differs in the length of the beak only. 



* Pleuroxiis prociirviis Birge. 

 Plate LXI, Figs. 3, 4. 



In size and general appearance this most interesting species is 

 similar to the above, and, especially, to P. denticulatus. The general 

 form and even the details of structure agree almost to identity with 

 the latter. The structure of the posterior margin is like FercanthOj 

 while the rostrum is bent abruptly upwards as in EhypopMlus. In 

 small individuals the length is greater proportionally. The lower 

 margin is slightly convex or nearly straight, and fringed by bristles 

 which are strongly pectinate; the anterior and lower margins are 

 toothed as in Pereantha. The shell gland is more as in the Daplinidce 

 than most Lynceids. The number of posterior teeth is variable. The 

 ephippium is as in P. denticulatus. Length 0.40 mm. to 0.50 mm. 

 Kot rare, but less common than P. denticulatus. The male post-abdo- 

 men is like that of P. denticulatus; the rostrum is as in the female. 



Of the species following it may suffice to say that they are corpu- 

 lent, filth-loving representatives of P. triffonellus, P. adunctus and P, 

 bairdii, respectively, which have turned up their noses at a superficial 

 existence and buried themselves in the mire and debris at the bottom 

 of the pools. It might be fanciful to assume that the curved snout is 

 used for "rooting," but the fact that these "Schmutzpeterchen" 

 Lynceids would find a long straight beak in the way is suggestive. 



