152 THE GASTROPODA 
sory organs are rarely found on the genital ducts, but are present in Palu- 
dina, Cyclostoma, the Naticidae, Calyptraeidae, etc. Mandibles are usually 
present. The intestine is long. The Platypoda form the largest group 
of the Mollusca, comprising nearly sixty families of unequal value, some of 
which are not thoroughly well known from an anatomical point of view. 
FamiLy 1. PALUDINIDAE, d’Orbigny, Ctenidium monopectinate ; pedal 
centres in the form of ganglionated cords; the kidney is provided with a 
ureter; viviparous ;_ fluviatile. Genera—Paludina, Lamarck.  Neo- 
thawma, Smith ; from Lake Tanganyika. Tylopoma, Brusina; from the 
Tertiary. Famriity 2. CychopHoripak, Gray. Pallial cavity devoid of a 
ctenidium and transformed into a lung; pedal centres in the form of 
ganglionated cords ; otocysts with otoconia; aperture of the shell and 
the operculum circular ; terrestrial. Genera—Pomatias, Hartmann ; shell 
turriculated. Diplommatina, Benson. Hybocystis, Benson. Cyclophorus, 
Montfort ; shell umbilicated, with a short spire and horny operculum. 
Cyclosurus, Morelet ; shell uncoiled. - Dermatocera, Adams; foot provided 
with a horn-shaped protuberance at its posterior end. Spiraculum, Pear- 
son ; aperture provided with a sutural tube at its superior angle. FAMILY 
3. AMPULLARIIDAE, Gray. A monopectinate ctenidium present, and to 
the left of it a pulmonary sac, separated from the ctenidium by an incom- 
plete septum ; oesophageal nerve-collar in front of the buccal bulb ; 
penis pallial; amphibious. Genera — Ampullaria, Lamarck ; visceral 
sac and shell coiled dextrally. Lanistes, Montfort ; shell sinistral ; spire 
short or obsolete. Meladomus, Swainson; shell elongated, sinistral. 
Faminy 4, Lirrortnipar, Gray. Ctenidium monopectinate ; oesophageal 
pouches present ; pedal nerve-centres concentrated ; a pedal penis near 
the right tentacle. Genera—Littorina, Férussac ; shell not umbilicated ; 
foot devoid of appendages ; marine forms of semi-aerial habit (Fig. 85). 
Lacuna, Turton ; foot with two posterior appendages ; marine forms of 
exclusively.aquatic habit (Fig. 7). Cremnoconchus, Blandford ; shell umbili- 
cated ; of exclusively aerial habit; Indian. Risella, Gray. Tectarius, Valen- 
ciennes, Faminy 5. FossaripAg, Fischer. Shell turbinated and umbili- 
cated ; head with two cephalic lobes, as in some Rhipidoglossa. Genus 
Fossarus, Philippi. Famriry 6. PuRPURINIDAR, Zittel. An exclusively 
fossil family ; shell thick, with prominent spire, angular whorls, and oval 
aperture. Genera— Purpurina, VOrbigny; Jurassic. Brachytrema, 
Morris and Lycett ; Jurassic. Sculites, Conrad ; Ordovician. FAmILy 7. 
PLANAXIDAE, Adams. Shell not umbilicated, with pointed spire; the 
external border of the shell sharp; a short pallial siphon. Genus — 
Planaxis, Lamarck. Faminy 8. CycLosroMaTIpAB, Pfeiffer. Pallial 
cavity transformed into a lung; pedal centres concentrated ; otocysts 
with otoliths ; no mandibles; a deep longitudinal pedal groove present ; 
terrestrial. Genera—Cyclostoma, Draparnaud ; shell turbinated ; opercu- 
lum calcareous; British. Omphalotropis, Pfeiffer; shell turriculated ; 
operculum horny. Faminy 9. AcicuLIDAE, Gray. Shell narrow and 
elongated ; operculum horny; pallial cavity a pulmonary chamber ; 
otocysts with otoconia. Genus— Acicula, Hartmann. Famiry 10. 
VALVATIDAE, Gray. Ctenidium bipectinate, free throughout its length ; a 
pallial filament on the right side; hermaphrodite ; fluviatile. Genus— 
