HYDEOBIA. 335 



Genus SKENEA, Fleming. 



Operculigerous lobe with small lateral wings and a 

 rudimentary posterior cirrhus. 



Operculum of few whorls, nucleus central. 



Shell orbicular, spiral, depressed and discoidal, deeply 

 urabilicated, few-whorled ; peritreme circular, continuous, 

 entire. 



Ex. S. planorbis, O. Fabricins, pi. 35, fig. 9. Oper- 

 culum, S. planorbis, fig. 9, a, 9, h. Shell, S. planorbis, 

 fig. 9, c, 9, d. 



This small genus, known by its planorbular, depressed 

 shell, has the eyes on bulgings at the outer bases of the 

 tentacles, and the opercular lobe as in Rissoa ; the other 

 small, depressed British shells, usually associated with it, 

 appear to belong to the genus Cyclostrema of Marryatt, 

 not of Fleming, and constitute the Delphinoidea of Brown. 



Genus HYDROBIA, Hartman. 



Operculigerous lobe simple. 



Operculum subspiral. 



Shell elongately conical, thin, smooth, covered with an 

 olivaceous epidermis, axis imperforate ; aperture oval, 

 peritreme continuous, outer lip acute, simple. 



Syn. Leachia, Risso, not Lesneur or JoJinst. Littori- 

 nella, Braun. 



Ex. H. ulvPB, Pennant, pi. 35, fig. 10. Operculum, 

 H. ulvffi, fig. 10, a, 10, h. Shell, H. ulvae, fig. 10, c. 



The species of this genus inhabit fresh or brackish 

 water and muddv estuaries ; the tentacles are subulate. 



