AND ALLIED FORMS. 17 



Paludina cincinnatiensis of Anthony. The Paludina ladrica 

 of Say, may perhaps belong to Gill's genus Lycxpjrus, one of the 

 Valvatidae, on account of its large umbilicus and labrum simply 

 touching the penult whorl — characters which recall the young 

 of Valvata pupoidca, Goukl. 



It may here be remarked that none of the so-called " Amnicolte," 

 the dentition of which is figured by Troschel, in his " Gebiss der 

 Schuecken," Vol. I, pi. viii, belong to the genus as here restricted 

 to forms congeneric with the type, A. limusa, Ilald. 



Genus BYTHISfELLrA, Moq.-Tand. 



Moquin-Tandon, in his work on the Terrestrial and Fluviatile 

 Mollusks of France, published in 1856, considered all the fresh- 

 water Rissoids of that country as belonging to a single genus, 

 B^thinia, which he divided into two subgenera : Fiona, 

 synonymous with the true Bytliinia of Gray, and Bythinella, 

 JVIoq.-Tand., comprehending numerous small snails belonging to 

 our subfamily Hydrobiin^. The preoccupied name Leachia 

 had already been applied to these latter forms by Risso. The 

 group Bythinella, as proposed by Moquin-Tandon, contains at 

 least two distinct genera, neither of which is synonymous with 

 our Amnicola ; but the name must be retained for the forms con- 

 generic v/ith B. viridis, the type of the genus as originally indi- 

 cated by that author in the "Journal de Conchyliologie," II (1851), 

 p. 239, note. I have not been able to procure specimens of these 

 European true Bythinella3 for examination in regard to their 

 lingual dentition ; but Troschel, in his work already frequently 

 quoted, has figm-ed the teeth of a species called by him "Amnicola 

 (Suhulina) thermalis,^'"- which is probably congeneric with the 

 true Bythinelloe, the Turbo thermalis of Linnteus being a fresh- 

 water species.^ In this species Troschel describes the rhachi- 

 dian tooth as having but one basal denticle on each side, and a 

 9-denticulated cusp ; the intermediate tooth with a peduncle longer 

 than the body, and a 6-denticulated cusp ; the inner lateral tooth 

 armed at the summit with eighteen teeth, and the outer one with 

 the margin of the summit smooth ; the character of this latter 



' Gebiss der Schnecken, I, 108, viii, 6. 



2 " Habitat prope thermas pisanas in aquis dulcibus." Linn., Syst. Nat., 

 ed. 12, No. 1237. 



May, 1865. 



