256 MANUAL OF THE MOLLTJSCA. 



most abundant in tlie north temperate zone. North xim erica, 

 West Indies, Norway, Britain, Mediterranean, Caspian, India, 

 &c. Rissoa parva adheres to sea-weeds by threads, like litiopa. 

 (Gray.) 



Fossil, 100 species. Permian — . Britain, France, &c. 



Sub-genera. Rissoina, D'Orbigny. Aperture channeled in 

 front. 66 living species. Fossil (10 species Bath ojlite. — • 

 Britain.) =^^T«65a, Lea? America. 



Hydrohia, Hartm. ( = Paludinella, Loven. ) Shell smooth ; foot 

 rounded behind ; operculigerous lobe without filament. Type, 

 littorina ulvse (PI. IX., Fig. 18). Distribution, b() s^QoiQS. Fossil, 

 10 species. Wealden — . Britain, &c. 



Syncera, Gray (Assiminea, Leach). S. hepatica. Shell like 

 Hydrobia ; tentacles connate with the eye pedicels, which equal 

 them in length. Teeth 5 — 7 cusped; uncini 1, 2, dentated, 3 

 rounded. Distribution, 2 species, brackish water. Britain and 

 India. 



Nematura, Benson. N. deltse (PI. IX., Fig. 21.) Aperture 

 contracted ; peristome entire ; operculum pauci-spiral. Fossil, 

 Eocene. Isle of Wight. 



Jeffrey sia, Alder (=Eissoella, Gray, MS.), J. diaphana. Shell 

 minute, translucent ; operculum semilunar, imbricated, with a 

 projection from the straight, inner side (PI. IX., Fig. 19). 

 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, onty visible 

 through the shell ; foot bi-lobed in front. 6 species. Britain. 

 On sea-weed, near low- water. (Alder.) There are eight other 

 species in the Japanese seas. 



Skenea, Fleming. 



Etymology, named after Dr. Skene, of Aberdeen, a contem- 

 porary of Linnsous. 



Synonym, Dclphinoidea, Brown. 



Type, S. planorbis (PI. IX., Fig. 20). 



Shell minute orbicular, depressed, few-whorled; peristome 

 continuous, entire, round ; operculum pauci-spiral. Animall^kQ 

 rissoa, foot rounded behind. Found under stones at low-water, 

 and amongst the roots of corallina officinalis. 



Distribution, ? species. Northern seas, Norway, and Britain. 

 S. cornuella, Straits of Korea (Adams). 



? Trtjncatella, Risso. Looping-snail. 

 Type, T. truncatula (PI. IX., Fig. 25). (Mus., Hanley ) 



