86 LAND AND FRESH WATER MOLLUSKS 



Section Par asp ir a Dall, nov. Shell resembling Diplodiscus, but 

 with the whorls rounded throughout, and the aperture simple, 

 hardly expanded. Type P. rotundatus Poiret. 



Subgenus Hippeutis Agassiz. Shell small, lenticular, with a small 

 number of rapidly increasing whorls, the last enveloping a large 

 part of the preceding whorl ; apex slightly depressed, base with a 

 narrow umbilicus, aperture oblique, with a thin sharp margin. Type 

 P.fontanus Lightfoot, European. 



Section Drepanotrema Crosse and Fischer. Shell resembling Hip- 

 peutis, but less depressed, the whorls with a rounded periphery 

 widest at the base, giving a domelike profile, umbilicus variable, 

 from narrow to very wide. Type P. yzabalensis C. and F. 

 This group occurs in the Antilles, Mexico, Central and South 

 America, where it represents the Palearctic Hippeutis. 



Section Menetus H. and A. Adams. Like Hippeutis, but the last 

 whorl not enveloping the preceding whorls to any marked extent. 

 Type Planorbis opercularis Gould. 



Section Batkyo7iiphalus Agassiz. Shell like Drepanotre?na, but with 

 numerous closely coiled whorls, a flattish summit with the periphery 

 nearer to it than to the base, the umbilicus moderate, exposing less 

 of the coil than the summit shows. Type P. contortus Drap. 



Subgenus Gyraulus Agassiz. Shell small, flattish, with few, rapidly 

 increasing whorls, fully exposed above and below, with a nearly 

 median periphery, spirally striate and hispid ; aperture simple, 

 sharp-edged, oblique. Type Planorbis albus Muller. 



Section Torquis Dall, nov. sect. Like Gyrauhis s. s. but with more 

 rounded, less rapidly increasing whorls, not hispid or spirally striate, 

 the aperture expanded and slightly thickened in the adult. Type 

 P. parvus Say. 



Section Armiger Hartmann. Shell small, with few, rapidly increas- 

 ing, costate whorls, the costae projecting at the periphery ; the form 

 in a general way like Gyraulus. 

 A discussion of the species follows. 



* Planorbis (Planorbina) glabratus Say. 



Planorbis glabratus Say, Journ. Acad. Nat. Sci., I, p. 280, 18 18 (South 



Carolina). 

 Planorbis lentus Say, Am. Conch., vi, pi. iv, fig. I, 1834 (New Orleans). 



This species is by no means always polished, and on a dull speci- 

 men of it I believe the later P. lentus Say to have been founded, 

 though the latter name has been widely misapplied to senile specimens 



