6o 



THE BRITISH NATURALIST. [March 



to regard such species as LimncBa stagnalis, Unto pictorum, &c., as 

 potentially polymorphic, that is, having a wide range of variabiHty, the 

 different phases of which are induced by environmental conditions. 

 The abihty to vary is inherited, and is, doubtless, of great value to the 

 species, but circumstances determine which varietal characters of 

 those possible shall appear in any particular individual. 



Planorhis. — P. lineatus is referred to P. nitidus, Mull., and a var. 

 clessini, Wst., is British. P. nitidiis of Jeffreys is P. coiuplanatns, L. 

 (non Jeff.) P. nautileus and crista are given as distinct species, as also 

 is P. drapaviialdi (our nlhus var.j P. spirorhis of Jeffreys is called P. 

 leucostoma, MelL, but ecarinatiis, Jeff., is considered a variety of the 

 true spirorhis, L. P. daziiri, Morch., is given as British. P. coniplan- 

 atns, Jeff., is called P.iimhilicatus, Mull., but according to strict priority, 

 it should apparently stand as Planorhis planorhis (L. ) P. elophilus, Bgt., 

 V. ammonoceras, Wst., is British ; we should consider it a variety of 

 Cornells. 



Physa. — P. fontinalis, v. inflata, is v. bnlla, Mull. 



Limncea. — L. involnta, L. hurnetti, L. ovata, and L. ampin, are con- 

 sidered valid species. L. auricularia, v. monnardi, is a form of amplay 

 and Jeffreys' acuta is referred to lagotis, Schr. L. stagnalis, v. elegans, 

 Leach, is described, and is, of course, British ; the recently-described 

 V. elegans, 'WWm?,. ("Midi. Nat.," 1889, p. 164), will apparently require 

 a new name. L. truncattda, v. turrita, CI., is quoted as British. 



Ancylus. — Our vars. captdoides and strictus oi A . fliiviatilis appear as 

 species. The white var. of the same species is called v. alhus, Stenz. 



Paludina. — P. vivipara, var. nevilli, Bgt., and P. S2ibfasciata , Bgt.^ 

 are British. 



Valvata. — V . piscinalis , Ya.Y. pusilla, appears as a species. V.flnvi- 

 atilis, Colb., is said to be English. According to Locard, Valvata 

 gallica, Loc, is found in England, but we call it merely piscinalis. 



Sphceri^im. — S. cornenm, v. flavescens, is referred to nucleus. S. 

 scaldianum appears as a species, with pisidioides. Gray, as a variety. 

 Two sub-species of S. rivicola are found in Britain ; »S. loirce, Bgt., and 

 5. gallicmn, 'Bgt., hoih near Manchester. Our S. ovale is called 5. 

 pallidum, Gray, a species distinct from true ovale. S. lacustre, v. 

 mamillare, Gass., is British ; and vars. hrochoniannm and ryckholtii are 

 placed as species. ^^iL4^ 



3?"AR.91 



