Synopsis Fresh-Water Mollusca 13 



Key to the sections of Gyranhis. 



1. Surface spirally striate and hispid Gyraidus s. s. 



2. Surface smooth or finely striate Torqiiis. 



3. Surface costate Armiger. 



Section GYRAULUS, s. s. /^<SX7 /^ 



Periphery rounded, or angulated, surface spirally 

 striate and hispid ; aperture simple, sharp-edged, oblique. 



Type : P. albus Miiller. 



Example: P. hirsutiis Gld., fig. 31. 



Fig. 31 



Section TORQUIS Dall, 1905. 



Like Gyraulus s. s., but with more rounded, less rapidly increasing 

 whorls, not hispid nor spirally striate ; aperture expanded and slightly 

 thickened in the adult. 



k 



Fig. 32 Fig. 33 



Type : P. parvus Say, fig. 32. Radula, fig. 33. 



Section ARMIGER Hartmann, 1840. 



Shell dextral, very small, with few rapidly increasing, K^-(^~^|vM 

 costate whorls, the costse projecting at the periphery; the 

 form in a general way like Gyraulus. 



Type : P. crista L., fig. 34- 



Fig. 34 



Genus SEGMENTINA Fleming, 1817. 



Shell discoidal, dextral, whorls few, rounded or carinated above and 

 below, with one or more sets of laminae or teeth in the interior of the shell. 



Animal as in Planorbis, tentacles filiform ; foot narrow anteriorly, wider 

 behind, obtusely rounded at both ends. Radula, central tooth bicuspid; 

 laterals tricuspid ; marginals 4-5 cuspid. ^ 



Type : Nautilus lacustris Lightfoot. : -Y^^^Jjk^ iAaaJlU^ 



