52 LECTURES ON MOLLUSCA. 



knowledge of the animal, however, it is impossible to say whether its 

 relationships are not rather with Nerita, or even with Natica. The 

 teeth in this family are not properly known. 



Family AMPULLARIADJB. (Apple- Snails.) 



The Apple-Snails form a very natural and peculiar group, standing 

 by themselves, and only presenting an external similarity to the other 

 fresh-water shells with which they are generally associated. They 

 inhabit the marshes of the tropical regions, both in the Old World and 

 the New, and are particularly fine and plentiful in Africa and South 

 America. They have a large globular shell, in some fossil species so 

 like Natica that it is hard to distinguish them. In general the shell 

 is thin, with a strong glossy skin and a horny operculum of concentric- 

 elements. Although there is no notch in the shell, the creature has 

 almost always a long breathing pipe, like that of the Whelks ; but 

 with this difference, that it is slit along the upper not the under side. 



The Apple-snails are truly amphibious, having, as it were, a gill in 

 the corner of a lung. This arrangement is necessary to enable them 

 to survive the long summer droughts, when they bury themselves deep 

 in the mud and wait for better times. They have been known to live 

 many years out of the water. Their eyes are of respectable dimen- 

 sions, planted on little pillars like the Strombs, with a pair of very 

 long, slender tentacles in front. There appears to be a second pair of 

 shorter tentacles in front of these, but they are really the two halves 

 of the muzzle which is split and lengthened out. The teeth are formed 

 on the tearing type of Natica, &c. The creatures are eaten in vast 

 numbers by marsh birds, who, if they cannot get at their prey through 

 the operculum, carry them up to the branch of a tree and break the 

 shell by the fall. 



In the true Ampullarias, which are peculiar to tropical America, 

 and are called " Idol-shells" by the Indians, the pipe is long and the 

 operculum horny. The group Pomella have thick, heavy shells, with 

 very wide mouth. In Marisa, which is found in the East Indies as 

 well as in America, the shell is flattened down till it resembles a Pla- 

 norbis. Lanistes, from the African rivers, has a flattened, reversed 

 shell. In Meladomus, also an African form, the spire is turreted, 

 looking like a reversed Paludina. In Pacliystoma, which includes 

 most of the old-world Apple-snails, the breathing pipe is short, and 

 there is a thickened ledge round the mouth, to support a somewhat 

 shelly operculum. In Asolene, which frequents the marshes of the 

 La Plata, there is no breathing pipe visible. The estuary species are 

 often found mixed with marine shells, both on existing shores and in 

 the tertiary beds. 



SECOND GROUP. Teeth arranged as for vegetable food. 



Among the land snails, there are some very beautiful tribes, almost 

 confined to the tropics and the warmer temperate regions, which can- 

 not be properly reckoned with the true pulmonate Gasteropods. In- 

 stead of a real lung, they have (so to speak) a gill-cavity %rmed for 

 air-breathing, left open by the mantle which is free from the nape of 



