1881.] NATURAL SCIENCES OF PHILADELPHIA. D3 



G. B. Sowerbj' , Jr., in comparing Planorbis with certain Am- 

 pullariffi {Marisa)^ says: " It is further to be remarked that the 

 discoidal Ampulhirife are dextral shells, and the Planorbes are 

 sinistral or reversed ; and although the latter are sometimes so flat 

 and orbicular that it is difficult to know which is the spiral side, 

 it may nevertheless always be ascertained b}' a careful examina- 

 tion. "i 



"While the anatomy of Planorhiii in its principal characters, is 

 presumed to be sinistral, and indicates, with the sum of other 

 characters (including habitat), a most intimate relationship to 

 P/njsa, which has, as is well known, a sinistral shell, yet some of 

 the authors who affirm the sinistral character of the soft parts or 

 body, say also, that the species of this genus {Planorbis)^ have a 

 dextral sliell, an inclusive and broad statement which applies to 

 all of the species, and apparently repeat this tradition, or else 

 assume that it is so, because the great majorit}' of gasteropodous 

 moUusks which have shells at all, have dextral shells, the excep- 

 tions being comparativeh' few. 



If we consider what are regarded as apical characters in forms 

 about which there is no question, and it is permitted to reason, 

 from analogy in this connection, it will be seen that some species 

 of Planorbis have sinistral shells, and I submit as examples 

 sustaining this position, the larger West American forms known as 

 P. amnion, (fig. 1) Gould (+ P. Traskii, Lea,), P. trivolvis, (fig. 2) 

 Say 2 ( 4- P. var. occidentalism Cp. MSS.), P. lumens^ Cpr., P. 

 subcrenafus, (figs. 3-3a) Cpr.^, P.corpulentus^ (figs. 4-4a) Say, and 

 P. tumidus, Pfr., from Nicaragua, also P. corneas. L., Britain, as 

 shown in authentic specimens received from an experienced and 



^ Concliological Manual, p. 345. 



- Pacific Coast specimens. 



^ This species more nearly resembles P. corneus, than does any other 

 American form. Many of the smaller so-called species (A.mevican) are 

 exceedingly close to the smaller forms of Europe, and it is not unlikely a 

 careful investigation would place some of them under the names previously 

 made by the earlier authors. Mr. W. G. Binney writes of Physa hypnorum, 

 "it is one of the species common to the three continents ;" and of Limmca, 

 he says, " It seems certain that the boreal regions are inhabited by several 

 species common to similar latitudes in A.sia and Europe, such as L. stag- 

 nalis and L. palmtris.^^ This remark will ultimately be found to apply 

 with equal truth to species of Planorbis. 



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