Synopsis Fresh-Water Moleusca 13 



Key to the sections of Gyraulus. 



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



2. Surface smooth or finely striate Torquis. 



3. Surface costate Armigcr. 



Section GYRAULUS, s. s. /::^J "'■i 



Periphery rounded, or angulated, surface spirally 

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



Type: P. alb us MxxW&r. 



Example: P. hirsutus 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. 



w 



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, 

 costate whorls, the costge projecting at the periphery; the 

 form in a general way like Gyraulus. 



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



Fig- 34 



Genus SEGMENTINA Fleming, 181 7. 



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

 below, with one or more sets of laminse 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: A' autilus lacustris L,\ghtioot. 



