GASTEROPODA. 137 



shaped. The}' range from high-water to 100 fathoms, but abound most in 

 shallow water, near shore, on beds oifuciis and zostera. 



Bistr., about 70 sp. Universally distributed, but most abundant in the 

 north temperate zone. N. America, W. Indies, Norway, Brit., Medit., Cas- 

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

 (Gray). 



Fossil, 100 sp. Pennian — . Brit., France, &c. 



Sul-genera. Rissoina, D'Orb. Aperture channelled in fi-ont. Li\ang and 

 Fossil (10 sp. Bath oolite.— Brit.)=r«^^, Lea? America. 



Hi/drobia, Hartm. (=Paludinena, Love'n. Paludestrina, D'Orb.) Shell 

 smooth ; foot rounded behind ; operc. lobe without filament. Type, litorina 

 ulvae, PI. IX., fig. 18. Fossil, 10 sp. Wealden— . Brit., &e. 



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. Distr., brackish water. 

 Brit., India. 



i\'m6r^«ra, Benson. N. deltte. PI. IX., fig. 21. Aperture contracted ; 

 peristome entire. Operc. pauci-spii'al. Fossil, eocene. Isle of Wight, 



Jefreysia, Alder (=Rissoella, Gray, MS.), J. diaphana. Shell minute, 

 translucent. Operc. semilunar, imbricated, with a projection fi'om 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, only visible 

 through the shell ; foot bi-lobed in front. 2 sp. Brit. On sea-weed, near 

 low water (xUder). 



Skenea, Fleming. 



Etym., named after Dr. Skene of Aberdeen; a cotemporary of Linnaus. 



Syti., delphinoidea. Brown. 



Type, S. planorbis, PI. IX., fig. 20. 



Shell minute orbicular, depressed, few-whirled; peristome continuous, 

 entire, round. Operc. pauci-spii-al. Animal like rissoa, foot rounded behind. 

 Found under stones at low- water, and amongst the roots of corallina offici- 

 nalis. 



Distr., ? sp. Northern seas. Noi-way, Brit. 



? Truxcatella, Risso. Looping-snail. 



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



Shell minute, cylindi'ical, truncated ; whirls striated transversely ; aper- 

 tm-e oval, entire ; peristome continuous. Operculum sub-spiral ! 



Animal ^ath short, diverging triangidar tentacles ; eyes centrally behind ; 

 head bi-lobed; foot short, rounded at each end (Forbes). 



The truncatellse are found on stones and sea- weeds between tide-marks, 

 and survive many weeks out of the water (Lowe). They walk by contracting 



