THE GASTROPODA 153 
Valvata, Miiller (Fig. 132); British, Famity 11. Rissorpar, Gray. A 
- monopectinate ctenidium ; epipodial filaments present ; one or two pallial 
tentacles; snout elongated. Genera — Rissoaw, Fréminville ; operculum 
simple. Rassoina, d’Orbigny ; operculum with an apophysis. Stiva, Hedley. 
Famity 12. Lrrroprpan, Fischer. Foot with an epipodium bearing three 
pairs of tentacles and an operculigerous lobe with two appendages ; in- 
habitants of the Sargasso weed. Genus—Litiopa, Rang. Faminy 13. 
ADEORBIIDAE, Fischer. Mantle with two posterior appendages ; ctenidium 
large and capable of being protruded’ from the pallial cavity ; shell 
depressed and umbilicated. Genus—Adeorbis, Wood ; British (Fig. 133). 
Faminy 14. JEFFREYSIIDAE, Fischer. Head with two long labial palps ; 
shell ovoid, umbilicated ; operculum horny, semicircular, concentric, and 
earinated. Genus—Jeffreysia, Alder. Faminy 15. HomanoGyriDag, 
Sars. Shell flattened ; operculum horny, circular ; no cephalic tentacles. 
Fic. 132. 
Valvata pisecinalis, dorsal aspect, Fic. 138. 
magnified. jf, foot; gi, gill; m, Adeorbis subcarinatus, ventral aspect. f, 
mouth ; op, operculum ; pa.t, pallial foot; g, gill; m, mouth; op, operculum ; 
tentacle ; sh, shell; te, cephalic pu.c, pallial cavity ; pa.t, pallial tentacle ; sh, 
tentacle. (After Bernard.) shell; t, tentacle. (After F. M. Woodward.) 
» Genera—Homalogyra, Jeffreys ; British. Ammoniceras, Vayssicre. FAMILY 
16. SKENEIDAE, Fischer. Shell depressed and umbilicated, with a 
rounded aperture ; cephalic tentacles long. Genus—Shkenea, Fleming ; 
British. Famriy 17. CHoristipak, Fischer. Shell spiral ; four cephalic 
tentacles; eyes absent; two pedal appendages behind the operculum. 
Genus—Choristes, Verrill. Faminy 18. ASsIMINEIDAE, Fischer. Shell 
conical, with a short spire; operculum horny, spiral; eyes situated at 
the free extremities of the tentacles. Genus—Assiminea, Leach ; estuarine ; 
British. Faminty 19.. TRUNCATELLIDAE, Gray. Ctenidium mono- 
pectinate ; snout very long, bilobed ; foot very short; spire elongated 
and truncated ; marine and littoral. Genus—TZruncatella, Risso. FAMILY 
20. Hyprosipasr, Fischer. Shell with prominent spire; ctenidium 
monopectinate ; sexes separate ; penis distant from the right tentacle and 
generally appendiculated ; otocysts with otoliths ; brackish water or 
fluviatile. Genera—Hydrobia, Hartmann; shell conical, smooth with 
scarcely convex whorls; operculum horny; brackish water; British. 
Baikalia, von Martens; from Lake Baikal. Pomatiopsis, Tryon ; foot 
