258 RISSOELLID^. 



Distr. — 26 sp. Ohio River and southwards to Alabama ; 

 Southern Atlantic States. A. tseniata^ Conr. (Ixxi, 11). 



Shell oval, heavy, with very short spire ; aperture entire and 

 rounded in front ; columella callously thickened above. 



MUDALiA, Hald. (Nitocris, H. and A. Ad.) Shell thinner, 

 inflated. Potomac and Susquehanna, Kanawha, and Upper Ohio 

 rivers. The distribution is thus more northern than that of the 

 type group. A. dissimilis, Say (Ixxi, 19). 



Family RISSOELLIDJE. 



Shell small, thin, transparent, spire elevated, aperture entire, 

 rounded or slightly sinuous anteriorly. Operculum corneous, 

 concentric. 



Animal with bilobate rostrum, eyes sessile on the head back 

 of the tentacles. 



These curious little animals are found adhering to floating sea- 

 weeds, in pools between tide-marks ; their eyes are situated so far 

 behind the tentacles that the transparency of the shell seems to 

 be essential to the vision of the animal. The bilobate mouth 

 and absence of retractile proboscis indicate them to be vegetable 

 feeders. In some respects closely allied to Litiopa. 



RissoELLA, Gray. 



Syn. — Jeflfreysia, Alder. 



Distr. — 6 sp. Britain. On sea-weed, near low-water. (Alder.) 

 There are eight other species in the Japanese seas. B. diaphana, 

 Forbes and Hanley (Ixxi, 20, 21). 



Shell minute, translucent. Operculum semilunar, imbricated, 

 with a projection from the straight, inner side. Head elongated, 

 deeply cleft, and produced into two tentacular processes ; mouth 

 armed with denticulated jaws, and a spinous tongue ; tentacles 

 linear, eyes far behind, prominent, only visible through the shell ; 

 foot bilobed in front. 



Fairbankia, Blanford, 1868. 



Distr. — F. Bombayana, Blf. Estuary ; Bombay Harbor. 



Shell imperforate, turreted, with a brown epidermis ; aperture 

 suboval, rounded in front; peristome slightly dilated, external 

 margin acute, but exteriorlj^ with variciform thickening. Oper- 

 culum corneous, subannular with an interior long, transverse 

 rib. 



Animal with long filiform tentacles, and eyes sessile at their 

 bases ; proboscis elongated ; foot wide and sinuated in front, 

 rounded behind. 



Combines the epidermis, and to a great extent the animal 

 of Hydrobia with the peristome of Rissoa ; the operculum 

 approaches that of Rissoella. It diliers from Barleeia in its 



