258 GEOL. AND NAT. HIST. SURVEY OF MINNESOTA. 
varies 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 Hurycercus and other 
Lyneeids, according to Schoedler. 
Pleuroxus truncata O. F. Miiller. 
Lynceus truncatus—Miiller, Koch, Zaddach, Lievin, Fischer, Lilljeborg, Leydig, Fric. 
Percantha 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 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 strix, 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. 
Percantha brevirostris Schoedler differs in the length of the beak only. 
* Pleuroxus procurvus Birge. 
PLATE LXI, Fiaes. 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 Percantha, 
while the rostrum is bent abruptly upwards as in Rhypophilus. In 
small individuals the length is greater proportionaliy. 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 Percantha. The shell gland is more as in the Daphnide 
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. 
Not 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. trigonellus, 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”’ 
- Iynceids would find a long straight beak in the way is suggestive. 
* , 7 ce yor) 
fae Ce ae GL Tee he eS, 
