HANNIBAL: CALTFORNIAN FRESHWATER MOU.USCA. 153 



frequently throughout life, axially and spirally striate, aperture 

 lunate, retracted above, comnionly simple, but dentate in one of the 

 modified groups; animal dextral, lieruiapliroditic, tentacles filiform, 

 buccal plate with accessory lateral jaws, foot quadrate ; habitat lakes 

 and streams. 



An examination of the early stages of the members of this family 

 reveals the fact that the old classification into Planorhis (edentate 

 species) and iSegmentina (dentate species) has no genetic significance, 

 since the smaller Planorbes with veitically compressed whorls and 

 more or less acute peripherally prove to be congeneric with Planorhis 

 {Segmentina) nitida, as suggested by Ficinus nearly fifty years ago, 

 constituting species which have either never developed apertural 

 teeth or in which they have been absorbed, while the large species, 

 such as Cornells, trivolvis, etc., belong to Ilelisoma. 



Both Planorhis and Ilelisoma in the primitive condition have 

 a plauulate spire and normal umbilicus, and each, tends to develop an 

 invaginate spire and planulate umbilicus to accommodate the dextral 

 animal. So many features of unequal parallelism appear, however, 

 that it has not been considered wise to attempt the recognition of 

 stages or sub-families without the study of additional genera. 



Genus Plaxorbis, Miiller. 



Helix (s-p.), Linne, 1758 {H. planorhis, L.); Planorhis {pars), Miiller, 

 1774 [P. carinatus, Miill. = H, planorhis, L., type by tautonomy) ; 

 Planorbarius, Dunieril, 1806 (emended form); Anisus (f;ffr«), 

 Studer, 1820 {P. Planorhis, L., type by substitution); Spirorhis, 

 Swainsou, 1840 {P. vulgaris, 8wains. = II. vortex, L.), not 

 Spirorhis, Daudin, 1800; Spiralina, Hartmann, 1840 (nude name); 

 Tropidiscus, Stein, 1850 {H. complanata, L.= H. planorhis, L.) ; 

 Gyrorhis, Moquin - Tandon, 1855 {P. carinatus, Miill. = 

 H. planorhis, L., type by substitution), not Gyrorhis, Fitzinger, 

 1833; Omalodiscus, Benson, 1855 (P. vulgaris, Swains. = 

 H. Vortex, L., type by substitution) ; Diplodiscus, Westerlund, 

 1897 {II. vortex, L.), not Diplodiscus, Diesing, 1850; Spiralina, 

 'Hartmann,' Von Martens, 1899 {H. vortex, L.); Paraspira, 

 Dall, 1905 {Planorhis rotundatus, Poir. = H. vortex, L.). 



Type, Helix planorhis, Linne. 



from simple patelliform ancestors, the Ancylidse illustrating how this has 

 taken place. Assuming that then, as now, the animal in each group was 

 dextral or sinistral as the case might be, while the shells were indis- 

 criminately sub-sinistral or sub-dextral, a ready explanation is afforded. 

 Once the evolution commenced toward the development of a spiral shell, 

 the position of the apex became a matter of the utmost importance, since 

 it determined absolutely whether the coil would be sinistral, dextral, or 

 enveloped by succeeding whorls ; consequently a sinistral or dextral, dextral 

 or sinistral shell is superimposed upon a sinistral or dextral animal for all 

 time. In case an unhappy combination resulted the only relief is in ultra- 

 sinistral or ultra-dextral growth to accommodate the animal. This latter 

 phenomenon is genetically distinct and readily detected as such. 



