GASTEROPODA. 137 



shaped. They range from high-water to 100 fathoms, but abound most in 

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



Dish'., 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. Permian — . Brit., Prance, &c. 



Sub-genera. Rissoina, D'Orb. Aperture channelled in front. Living and 

 Fossil (10 sp. Bath oolite. — '!^x\i.)=^Tiiha, Lea? America. 



Hifdrobia, Hartm. ( =PaludineUa, Loven. Paludestrina, D'Orb.) Shell 

 smooth; foot rounded behind ; operc. lobe without filament. 2]/^;^, litorina 

 ulvffi, PL IX., fig. 18. Fossil, 10 sp. Wealden— . Brit., &c. 



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

 tentacles connate Avith the eye pedicels, which equal them in length. Teeth 

 5 — 7 cusped; unciui 1, 2, dentated, 3 rounded. Distr., brackish water. 

 Brit., India. 



Nemaiitra,'S>ensoJi. N. delt^e. PL IX., fig. 21. Aperture contracted; 

 peiistome entire. Operc. pauci-spiral. Fossil, eocene. Isle of "Wight. 



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

 translucent. Operc. 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, only visible 

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

 low water (Alder). 



Skenea, Fleming. 



Ftpn., named after Dr. Skene of Aberdeen ; a cotemporaiy of Linnseus. 



Syn., delphinoidea, Brown. 



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



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

 cntii'e, round. Operc. pauci-spiral. Animal lOce rissoa, foot rounded behind. 

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

 nalis. 



Distr., ? sp. Northern seas. Norway, Brit. 



? Truncatella, Eisso. Looping-snail. 



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



'Shell minute, cylindiical, truncated ; whirls striated transversely ; aper- 

 tui'c oval, entire ; peristome continuous. Operculum sub-spiral ! 



Animal with short, diverging triangular tentacles ; eyes centrally behind ; 

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



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

 and sm-vive many Avecks out of the water (Lowe). They walk by contracting 



