AND ALLIED FORMS. 43 



that species is decided with some degree of certainty, materials 

 are not always at hand for the determination of the important 

 characters of its soft parts. 



HYDROBIA, Haktmann. 



Ilydrohia, Hartmann, in Sturm's Fauna Deutschland's, Abth. VI, (1821,) 



Heft 5, p. 46 (in part). 

 Paludinella, LovkN, Ofv. af. k. vet.-Akad. Forh., Ill, (1846,) 157 (not 



of Pfeifkek). 

 Littorinida, Eyd. & Soul., Voy. Bonite., Zool., II, 536. 



Shell ovate or elongated, smooth, subperforate ; spire conic ; 

 whorls flat ; apex acute ; aperture ovate ; inner lip not thickened. 

 Operculum corneous. Rostrum rather long. Tentacles some- 

 what tapering, but blunt at the extremity. Foot somewhat 

 pointed behind. 



Lingual dentition of the type : Rhachidian tooth very broad, 

 with a strong, central, obtuse, tongue-shaped process from the 

 anterior concave surface^ directed downward and reaching beyond 

 the base. Intermediate tooth shaped as in Littorinella minuta, 

 •"but with a deep concavity in the body. Lateral teeth with the 

 dorsal or exterior margin of the peduncle or shank reflected or 

 thickened. Formula of the denticles : yti - 6 - 13 - 25. 



Station, brackish water. 



Distribution, mundane. 



Type H. nlvm, H. & A. Ad., Gen. Rec. Moll., I, (1854) 335, xxxv, 10 

 (animal, shell, and operculum). — Turbo ulvce, Penn. — Rissou tdvce, Forbes 

 & Hanl., Brit. Moll., Ill, (IS.'i:'.) 141, Ixxxi, 4, 5, 8, 9, and Ixxxvii, 2, 8 

 (shell), and jj, 8 (animal), llab, Europe. 



The difficulty of separating the Littorinellae and Hydrobise 

 from the Bythinella3 and Paludestrina3 by the shell alone, has been 

 already alluded to. It might be convenient, as a temporary ex- 

 pedient, to arrange all the brackish-water species in the two 

 former genera, and the fresh-water ones in the other two, the 

 ultimate separation being founded upon the character of the apex 

 of the shell and of its whorls. 



My reasons for retaining the name IJydrohia for the genus 

 typified by H. idvae, have been stated on page 6. 



The Paludinella of Loven (not of Pfeif., on which see p. 18) 



