GENUS LIMNEA (Pond Snails). The generic word is derived from the Greek, limne, a marsh or pool. The animal is of a greyish colour; the head is short and broad, with two flattened short triangular tentacles bearing eyes at their bases; the foot is broad and short, with two lobes, or simply notched in front. The jaw is composed of three smooth pieces; the superior one is usually produced in front to form a slight beak. The general character of the lingual ribbon is such as is represented in fig. 24. The shell is spiral, oval, or oblong, thin, fragile, and translucent; the last whorl is large; the aperture is longer than wide, oval, with a thin edge and an oblique fold on the columella. They inhabit still and shallow waters, crawl slowly, float along the surface of the water with the foot, in the fashion of a boat, and the shell downwards, for the purpose of supplying themselves with air and collecting food. In a state of repose they adhere by their foot to stones and plants, and are capable of long immersion; in drought they partially bury themselves in the mud. The pond snails make a very audible squeaking noise on being taken out of the water. This arises from the expulsion of the water as the animal retreats within its shell. The food of the pond snails is animal and vegetable matters in different states of putridity; they also feed on living aquatic plants, and the green confervæ encrusting their shells have been observed to be objects of attraction among themselves. Dr. Bland noticed that the water snails, by cleaning off the algal growth of the shells of their neighbours, removed the epidermis, or even made holes in them by this continued rasping; and thereby accounted for the decollation of the upper whorls of their shells, when not attributable to chemical agencies; formerly this propensity was regarded as one of true cannibalism, because, in the absence of other food, the snails devoured each other by piercing the shell at the apex, and eating away the upper parts of its inhabitant. The water snails are very important elements in an aquarium, where the removal of decaying vegetable matter is necessary; they cannot, however, keep in check the confervoid growth. Eight species of the genus are known in Great Britain, and are contained in three sections: I. L. peregra, L. auricularia, and L. stagnalis, have the last whorl much enlarged. II. L. palustris, L. truncatula, and L. glaber, form another section, in which the spire is much elongated and the whorls gradually increase in size. III. L. glutinosa and L. involuta are separated by some authors from the true pond-snails, and placed in a sub-generic group under the name of Amphipeplea, because they possess a globular membranaceous shell, and the animal, though like Limnæa, has the edges of the mantle, when the snail is in motion, extended so as to cover the shell. A Limnoa is known in the Middle Purbecks, and the genus is represented by numerous species in the fresh-water beds of the Upper Eocenes, in the Isle of Wight; of the living species, L. palustris, L. peregra, and L. truncatula, first appeared during the deposition of the Mammaliferous Crag at Bramerton and Bulcham; they, with L. stagnalis and L. auricularia, occur in the Pleistocene marls of Essex. LIMNEA PEREGRA—(the Wandering Mud Snail) (Pl. X., fig. 117).-This mollusk is the most widely dispersed and abundant, and, at the same time, the most variable of the fresh-water snails. The shell is ovate, thin, the colour varying I. L. peregra, L. auricularia, and L. stagnalis, have the last whorl much enlarged. II. L. palustris, L. truncatula, and L. glaber, form another section, in which the spire is much elongated and the whorls gradually increase in size. III. L. glutinosa and L. involuta are separated by some authors from the true pond-snails, and placed in a sub-generic group under the name of Amphipeplea, because they possess a globular membranaceous shell, and the animal, though like Limnoa, has the edges of the mantle, when the snail is in motion, extended so as to cover the shell. A Limnoa is known in the Middle Purbecks, and the genus is represented by numerous species in the fresh-water beds of the Upper Eocenes, in the Isle of Wight; of the living species, L. palustris, L. peregra, and L. truncatula, first appeared during the deposition of the Mammaliferous Crag at Bramerton and Bulcham; they, with L. stagnalis and L. auricularia, occur in the Pleistocene marls of Essex. Limnæa peregra—(the Wandering Mud Snail) (Pl. X., fig. 117).-This mollusk is the most widely dispersed and abundant, and, at the same time, the most variable of the fresh-water snails. The shell is ovate, thin, the colour varying |