256 MANUAL OF THE MOLLUSCA. 
most abundant in the north temperate zone. North America, 
West Indies, Norway, Britain, Mediterranean, Caspian, India, 
&e. Rissoa parva adheres to sea-weeds by threads, like litiopa. 
(Gray. ) 
Fossil, 100 species. Permian—. Britain, France, &c. 
Sub-genera. LRissoina, D’Orbigny. Aperture channeled in 
front. 66 living species. Fossil (10 species Bath oolite.— 
Britain.) = Tuba, Lea? America. 
Hydrobia, Hartm. (=Paludinella, Loyén.) Shell smooth ; foot 
rounded behind; operculigerous lobe without filament. Type, 
littorina ulvyee (Pl. [X., Fig. 18). Distribution, 50 species. Fossil, 
10 species. Wealden—. Britain, &c. 
Syncera, Gray (Assiminea, Leach). 8. hepatica. Shell like 
Hydrobia; tentacles connate with the eye pedicels, which equal 
them inlength. Teeth 5—7 cusped; uncini 1, 2, dentated, 3 
rounded. Distribution, 2 species, brackish water. Britain and 
India. 
Nematura, Benson. N. delte (Pl. IX., Fig. 21.) Aperture 
contracted; peristome entire; operculum pauci-spiral. fossil, 
Eocene. Isle of Wight. 
Jeffreysia, Alder (—Rissoélla, Gray, MS.), J. diaphana. Shell 
minute, translucent; operculum semilunar, imbricated, with a 
projection from the straight, inner side (Pl. 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, only 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 Linneeus. 
Synonym, Delphinoidea, Brown. 
Type, S. planorbis (Pl. IX., Fig. 20). 
Shell minute orbicular, depressed, few-whorled; peristome 
continuous, entire, round; operculum pauci-spiral. Animal like 
rissoad, 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). 
? TRUNCATELLA, Risso. Looping-snail. 
Type, T. truncatula (Pl. [X., Fig. 25). (Mus., Hanley.) 
