BTTHINELLA. 6t 



sinuses. Rliacliidian tooth of the lingual ribbon with the basal 

 teeth on the inferior margin. Size small. Marine. Genera JRissoa, 

 Frem., Cingida, Flem., Alvania, Risso, Onoba, 11. & A. Ad., 

 Setia, H. & A. Ad., Ceratia, H. & A. Ad. 



Skenein.^;, with a d£pressed, almost discoidal shell, and a 

 corneous, paucispiral operculum. Minute. Marine. Genus 

 Sfcenea, Flem. 



HYDROBiiN-ffi, with shell and operculum and foot like those of 

 the Rissoinse, but with the rhachidian tooth of the lingual ribbon 

 having the basal teeth on the anterior surface, behind the lateral 

 margins. Size variable ; some are minute, some as large as 

 Bythinise. Living in fresh or brackish water. Genera Hydrohki, 

 Hartm., LittorineUa, Braun, Amnicola, Gould & Hald., Bythi- 

 nella, Moq.-Tand., Stenothyro,, Benson, Tricula, Benson, Pyr- 

 gula, Christ. & Jan, Paludestrina, D'Orb., Tryonia, Stm., Pote- 

 mopyrgus, Stm., Lithoglyphus, Muhlfeldt, Fluminicola, Stm., 

 Gillia, Stm., Somatogyrus, Gill, Cochliopa, Stm. 



PoMATiopsiN^, with the shell and operculum as in the Bis- 

 soinae. Foot with lateral sinuses. Size small. Amphibious. 

 Genus Pomatiopsis, Tryon. 



The land and fresh-water species only are included by me in 

 the following pages. The figures are all somewhat enlarged. 



BYTHI]¥EI.LA, Moq.-Tand. 

 Lingual dentition of B. thermalis, according to Troschel : 

 Rhachidian tooth moderately long, with the infero-lateral angles 

 much produced. Intermediate tooth with the body longer than ^ 



Fig. 131. 



Lingual dentition of Bythinella mcWmiaMa.— [Stimpson.] 



broad. Formula of the denticles : j|j - 6 - 18 - 0. Tentacles taper- 

 ing, but blunt at tip. Foot rather narrow, rounded behind. 



