1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 167 
central teeth are wider than long; laterals with a long rhomboidal body and several 
cusps, of which the second is much the largest. The two marginal teeth are bicuspid 
or rarely tricuspid. 
The animal is amphibious. When it is completely immersed, respira- 
tion may be wholly branchial; when out of the water, directly pulmonary. 
Ordinarily it obtains air by occasional visits to the surface, exactly like a 
lymneeid snail. 
The globular eggs have a calcareous shell, and are deposited in 
clusters on reeds some inches above the surface of the water. 
The family inhabits the fresh waters of the tropics of both hemi- 
spheres. It is entirely absent from the Palearctic Region (except Lower 
Kgypt). In North America it extends from Central Mexico to the La 
Plata system; a few species occur in Florida and Georgia, and some 
others are found in the West Indies. Most of the species prefer marshy 
low banks of rivers, swamps, ponds, or lakes to running water. Some of 
them are able to stand prolonged desiccation. 
The classification is based chiefly upon characters of the shell and 
operculum, since the soft parts are still unknown in a majority of the 
species, among them all those belonging to the groups Saulea, Afropo- 
mus, and Limnopomus. The interrelations and, to some extent, the 
limits of genera are not well understood. . The major divisions of the 
following key to genera are artificial, for want of knowledge of the three 
groups mentioned above. 
By external characters these snails may be grouped primarily into 
longisiphonate forms, including the American groups Pomacea and 
Ceratodes, and ‘brevisiphonate, comprising the American Asolene and 
Pomella, and the Old World genera Pila and Lanistes with its subgenera 
Meladomus and Leroya. 
The radula (Fig. 13) is singularly uniform throughout the family, 
and at present it appears doubtful whether it differs in any constant 
features in the three main genera, Pomacea, Lanistes, and Pila.? 
A few of the smaller groups or species have certain peculiarities, such as 
the wide marginal tooth and the produced lower angles of the central in 
Lanistes (Leroya) grauert, and the reduction of all but the main cusps in 
the teeth of Pila. (Turbinicola) nux.? 
1This is the case with Pomacea paludosa (Say), which we have had under observation in the 
aquarium. When obtaining air, the animal is always partially supported by floating vegetation and 
remains wholly immersed, only the left siphon protruding above water. A. genesensis (Deshayes) in 
an aquarium was seen to take air only twice in over six months. A review of the mechanism of respira- 
tion in Ampullariide is given by F. Babdk, 1921, in H. Winterstein, ‘Handbuch der Vergleichenden 
Physiologie,’ I, 2, pp. 540-542. 
2 Troschel, 1856-1863, ‘Das Gebiss der Schnecken,’ I, pp. 86-90, has made practically the same 
observations. ; ne 
3H. Burrington Baker (1922, Occas. Pap. Mus. Zool. Univ. Michigan, No. 106, p. 38) found a tend- 
ency to double the cusps by division in Ampullarius flagellatus. In a few cases the central tooth had as 
many as 11 cusps, the normal number being 7. 
