GENERA AND SPECIES. 



^ x ^ in. shell round, anterior side truncate and 



rounded, posterior side produced and 



sloping abruptly. 

 X in. shell oval, both sides rounded and 



compressed, 

 f- X ^jy in. shell oblong, anterior side truncate and 



sloping abruptly below, posterior side 



produced and rounded. 



There are varieties of all the species olPisidittm except roseitw, the most marked being 

 henslowana of 86, which hai a plate-like appendage to the beaks. 



Planorbis. (LIMN&ID&.) Plate xvii. 



Last whorl embracing the rest 



134 



87. nitidum, 



88. pusillum, 



89. roseum, 



222. lineatus, 

 224. nitidus, 



in. whorls 4, septa in last whorl. 



in. whorls 4 or 5, thin, shining, compressed, 

 keeled in middle. 



Last whorl not embracing the rest 

 Whorls rounded 



215. albus, 



218. contortus, 



219. corneus, 



221. glaber, 



216. carinatus, 



217. complanatus, 



220. dilatatus, 

 223. nautikus, 



225. spirorbis, 



226. vortex, 



^ in. whorls 5, shell small, greyish white, covered 

 with longitudinal strise ; last whorl large. 



^ in. shell small, thick, whorls 8, compact, mouth 

 semilunar ; umbilicus large and deep. 



i in. whorls 5 or 6, rounded above and below, 

 suture deep, mouth narrow and crescent 

 shaped ; umbilicus large and deep. 



\ in. whorls 5, last whorl large ; shell convex 

 above, concave below ; smooth, no circular 

 striae ; umbilicus large. 



Whorls keeled or angulated 



J in. whorls 5 or 6, keel in middle of whorls, last 

 whorl large, mouth obliquely oval. 



| in. whorls 5 or 6, keel on lower side of whorls, 

 last whorl large, mouth rhombic. 



^ in. whorls 2, angulated, mouth very large and 

 squarish, umbilicus small and deep. 



jLj in. whorls 3, last whorl rather large, small 

 imbricated ridge, mouth oval, umbilicus 

 large. 



in. whorls 5 or 6, one side concave the other 

 flat, keel frequently blunt, mouth roundish, 

 often ribbed with white, umbilicus wide and 

 shallow. 



g in. whorls 6 to 8, upper side concave, under- 

 side flat, keel on the lower margin and 

 always well marked, mouth rhombic, 

 umbilicus large and shallow. 



Among the noticeable varieties of Planorbis, zrecristata of 223, in which the periphery is 

 deeply notched; compressed of 221, which is only depressed in the centre on the upper side; 

 and coinpressa of 226, which has the keel nearly in the middle of the periphery. Nearly 

 every species has a white variety. 



