THE GASTROPODA 



'53 



Valvata, Miiller (Fig. 132); British. FAMILY 11. RISSOIDAE, Gray. A 

 monopectinate ctenidium ; epipodial filaments present ; one or two pallial 

 tentacles ; snout elongated. Genera Rissoa, Fre"minville ; operculum 

 simple. Rissoina, d'Orbigny ; operculum with an apophysis. Stiva, Hedley. 

 FAMILY 12. LITIOPIDAE, 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. FAMILY 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). 

 FAMILY 14. JEFFREYSIIDAE, Fischer. Head with two long labial palps ; 

 shell ovoid, umbilicated ; operculum horny, semicircular, concentric, and 

 carinated. Gemis Jejfreysia, Alder. FAMILY 15. HOMALOGYRIDAE, 

 Sars. Shell flattened ; operculum horny, circular ; no cephalic tentacles. 



Fio. 132. 



Valvata piscinalis, dorsal aspect, 

 magnified. /, foot ; gi, gill ; m, 

 mouth ; op, operculum ; pa.t, pallial 

 tentacle ; sh, shell ; te, cephalic 

 tentacle. (Alter Bernard.) 



Flo. 133. 



Adeorbis siHmirinatus, ventral aspect. /, 

 foot ; <7, gill ; m, mouth ; op, operculum ; 

 pa.c, pallial cavity ; pa.t, pallial tentacle ; sh, 

 shell ; t, tentacle. (After F. M. Woodward.) 



Genera Homalogyra, Jeffreys ; British. Ammoniceras, Vayssiere. FAMILY 

 16. SKENEIDAE, Fischer. Shell depressed and umbilicated, with a 

 rounded aperture ; cephalic tentacles long. Genus Skenea, Fleming ; 

 British. FAMILY 17. CHORISTIDAE, Fischer. Shell spiral ; four cephalic 

 tentacles ; eyes absent ; two pedal appendages behind the operculum. 

 Genus Clwristes, Verrill. FAMILY 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. FAMILY 19. TRUNCATELLIDAE, Gray. Ctenidium mono- 

 pectinate ; snout very long, bilobed ; foot very short ; spire elongated 

 and truncated ; marine and littoral. Genus Truncatella, Risso. FAMILY 

 20. HYDROBIIDAE, 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, Hartmaun ; shell conical, smooth with 

 scarcely convex whorls ; operculum horny ; brackish water ; British. 

 Baikalia, von Martens ; from Lake Baikal. Pomatiopsis, Tryon ; foot 



