HANNIBAL: CALIFOKNIAN FRESHAVATER MOLLUSCA. 155 



whorl, the other tlie columella ; Gyraulus stage passed during early 

 adolescence, Planorhis stage pushed back to earliest post-larval growth ; 

 habitat similar to preceding. 



Sub-genus Hippeidis. Shell similar to preceding but lacking the 

 denticulations (which have been absorbed), whorls usually fewer, 

 spire-pit generally narrower and deeper, umbilicus nearly enveloped ; 

 Segmentina stage apparently passed fairly early during adolescence ; 

 habitat same as preceding. 



Planorbis has probably suffered worse vicissitudes than any other 

 genus treated in these pages. Not only has it been incorrectly 

 divided, but the group to which the name has been restricted in 

 recent literature belongs to another genus. Miiller instituted Planorhis 

 in 1774, ostensibly to receive the present families Planorbidfe and 

 Physidfe. No type was designated, since it was not then customary, 

 but the following species were described: — 



Section *, shell depressed. 



Planorhis contrariitK, Miill. {Helix cornu-arietis, L., renamed) = JIarissa 



cor7it(-arietis (L.), South American. 

 P. ptirpura, Miill. (If. cornea, L.) = Helisnma cornea (L.). 

 P. carinatus, Miill. {U. planorhis, L.) = P. planorhis (L.). 

 P. vortex, Miill. {H. vortex, L.). 



P. umhilicatus, Miill. {IT. complanatus, L.) = P. planorhis (L.). 

 P. spirorbis, Miill. [IT. spirorbis, L.) = P. vortex (L.). 

 P. contortus, Miill. {II. contortus, L.). 

 P. nitidus, Miill. 

 P. albus, Miill. 



P. imhricatits, Miill. (JVautilus crista, L.) = P. crista (L.). 

 P. similis, Miill. = Uelisoma cornea (L.), juvenile. 



Section **, shell conical. 



P. bulla, Miill. {Btilla fontinalis, L.) = Physa fontinaUs (L.). 

 P. turritus, Miill. {B. hypnorum, L.) = Physa hypnorum (L.). 

 P. gelatinus, Miill. = 'i Physa fontinaUs, L. 



The second section was removed by Miiller to Bulinus the next 

 year. Lamarck, 1799 and 1801, cites the Marissa, an Ampullaroid, 

 the anatomy of which was then unknown, but whicli has since proved 

 to be very different from that called for in Miiller's diagnosis, hence 

 it must be excluded. Swainson (1840), the Adams (1855), and Tryon 

 (1884) give the second species, Helix corneus of Linue, as an example, 

 and Dall (1905) cites this as the type. If this were the sum and 

 substance of the matter their example must be followed, and Planorhis 

 of future usage, like Planorhis of the last seventy years, would be 

 a very different thing from Planorhis of Miiller, including but two of 

 tlie original fourteen species, and one of them regarded by the author 

 as doubtfully distinct. Now seven (five, omitting synonyms) of 

 Miiller's species are congeneric with H. 2>l(inorhis of Linne, and one is 

 proposed as a substitute for it, the author merely following a current 

 practice of the time of altering the specific name when it became 

 necessary to place the species in a new genus. It seems perfectly 



