84 LAND AND FRESH WATER MOLLUSKS 



is, of course, absurd. The first author to introduce Planorbis to 

 binomial literature was O. F. Muller, but as he used it, it was applicable 

 to all aquatic Pulmonates with filiform tentacles, thus including Physa. 

 Cuvier in 1798 cited three species, of which P. corneus was the first 

 and P. cornu-arietis the second. In 1799 Lamarck cited the second 

 species, and repeated this citation in 1801. But P. cornu-arietis, of 

 which the soft parts and operculum were then unknown, does not 

 agree with Lamarck's diagnosis and cannot be accepted as a type of 

 the genus Planorbis. Draparnaud names no types, and only in 1810 

 does Montfort establish definitely the type of the genus as P. corneus. 

 Subsequent selection of other types by later authors is, of course, of 

 no effect. Anisus Studer was an exact synonym of Planorbis Mul- 

 ler, not Montfort, but Fitzinger, in 1833, made an effort to retain the 

 name for a section of the true Planorbes. It is probable, however, that 

 the name, which was intended to comprise two older genera already 

 named, should be entirely eliminated from accepted nomenclature, as 

 a gross violation of the rules. At any rate the name was used for 

 a beetle by Dujardin in 1821, and no subsequent use of it is advisable 

 for Mollusks. 



Little is known of the anatomical characteristics of the various 

 species, but much similarity is noted among those for which data are 

 available. As to the shells, a wide variation is observable in form and 

 sculpture, though as usual the peripheral species grade toward each 

 other in the several groups. The great majority of the species present 

 the peculiarity of the whorls rising above the original apex, which 

 becomes basal, the shell (apparently sinistral) thus becoming what has 

 been called ultra-dextral. The most obvious characters of the latter 

 may be contrasted as follows : 



Subgenus Planorbis s. s. Shell discoid, ultra-dextral, large, with a 

 moderate number of gradually enlarging whorls rounded above and 

 below ; the aperture slightly and gradually expanded, with its margin 

 simple and sharp. Type P. corneus Muller. 



Section Planorbina Haldemann. Shell like Planorbis s. s. but verti- 

 cally compressed, with smaller and more numerous whorls and a 

 very oblique aperture. Type P. olivaceus Spix. 



Subgenus Helisoma Swainson. Shell of moderate size, few whorled, 

 the whorls carinate above and below and rapidly enlarging ; spire 

 and base funicular, aperture suddenly expanded, with a thickened 

 peritreme. Type P. bicarinatus (Say) Sowerby. 

 There is no doubt that Sowerby's and Say's species are identical. I 

 am unable to find distinctive characters in figures or diagnoses which 



