54 Our British Snails 



Planorbis contains the sub-genera Hippeutis, 

 Gyraulus, Gyrorbis, Coretus, and Bathyomphalus. 



Planorbis [Hippeutis] fontanus is much like 

 Segmentina but has no septa, and is flatter. 

 Common, especially on watercress. Often encrusted 

 with mud. 



Planorbis [Gyraulus) nauttleus is very small ; 

 quoit-shaped, with the upper side flat. Grey and 

 striated. The variety crista has the ridges of 

 the epidermis drawn into points, and is beautiful 

 when seen by a magnifying glass. Common in 

 ponds and ditches. 



Planorbis [Gyraulus) dilatatus is a very small 

 shell imported in cotton bales from America, 

 and naturalized in canals in Lancashire. No 

 other of its kind is so small. 



Planorbis [Gyraulus) albus is dull white and stri- 

 ated. Flattish above, with spire depressed. Fre- 

 quently encrusted and black with mud. Common . 



Planorbis [Gyraulus) parvus (but not so small 

 as dilatatus). — Convex above with a central de- 

 pression, concave beneath. Suture deep, and 

 umbilicus large. Smooth and glossy. Local. 



Planorbis [Gyrorbis) spirorbis.- — Very flat, glossy, 

 brown, whorls 5 to 6. Common in ponds and 

 ditches. 



Planorbis [Gyrorbis) vertex. — Very like the last, 

 but flatter and thinner, and with a prominent 

 keel. More local than spirorbis, but sometimes 

 found with it. Whorls 6 to 8. 



