INVERTEBRATE PALEONTOLOGY. 571 



RISSOID^E. 

 Genus HYDROBIA, Hartmann. 



Synon. — Hydrobia, Hartmann (1821), in Sturm's Fauna Deutschl., Abtb. VI, Heft V, 4(5; and in Steiiim. 

 Neue Alpen, I, 258 ; also (1840) Syst. Uebors., 258 (as a subgenus under Paludina). — 

 Gray (1840), Tnrton's Man. (2d ed.), 87 aud 88.— H. and A. Adams (1855), Genera 

 Recent Moll., I, 335.— Chenu (1859), Man. Conch., I, 303.— Stimpson (1865), Researches 

 upon the Hydrobiinw, iu Smithsonian Miscellaneous Contributions, 43. 

 Paludinella, Loven (1846), Ofv. af K. Vet.-Akad. Forh., Ill, 157 (not Pfeiffer). 

 Littorinida, Souleyet (1852), Voy. Bonite, Zool., II, 536. 

 Etym. — i dap, water ; Jiou, to live. 

 Type.— Turbo ulvas, Penn. (= Hyd. iilra*, H. and A. Adams). 



Shell small, subovate or more or less elongated, with axis generally 

 perforated or subperforated ; spire conical ; volutions flattened or convex ; 

 apex acute ; aperture ovate ; inner lip thin ; outer lip acute, simple ; surface 

 smooth; operculum corneous and subspiral. 



This genus includes a group of very small brackish-water species that 

 are generally difficult to distinguish from several other allied genera by the 

 , shell alone. From BytMnetta and Paludistrina, they differ in their brackish- 

 water habits. Littorinella, however, is also a brackish-water type, with a 

 shell very similar to that of Hydrobia, but usually has more convex or ven- 

 tricose whorls. There are also differences in the dentition and some of the 

 anatomical details of the animal, but such characters are of no use to the 

 Palaeontologist. 



The geological range of this genus is not well determined. Species 

 have been referred to it from the Wealden and some older rocks ; and others 

 apparently belonging to it occur in the Tertiary strata of various ages. It 

 probably attains its greatest development at the present time. 



Hydrobia A n t h o n y ■ , M. & H. 



Plate 43, figs. 10, a,b,c,d. 



Melania Jnthonyi, Meek aud Haydeu (1856), Proceed. Acad. Nat. Sci. Philad., VIII, 124; and (1860) ib., 



XII, 430. 

 Hydrobia Anthonyi, Meek (1866), in Conrad's Smithsonian Check-List N. Am. Eocene Invertebrate 



Fossils, 12. 



Shell small, conic-ovate ; spire rather short, pointed at the apex ; volu- 

 tions five and a half to six, a little convex, and increasing rather gradually in 

 size, last one not much enlarged, subanijular near the middle ; suture rather 

 well defined, but not deep; surface nearly smooth, or usually only marked 

 by very small obscure lines of growth ; aperture ovate, approaching rhombic- 



