The limits of age of molluscs has been definitely ascertained in a few instances only. Most Land-Snails probably live about two years, although in confinement some have been kept alive for a much longer time. Some of the marine forms live for a considerable period, the Common Oyster not attaining full growth until about five years old, after which it may continue to live for many years. The Giant Clam, a specimen of which is placed in the upright cases near the entrance to the Gallery, must, one would think, have a very long existence, judging from the size and thickness of the shell. [Sidenote: Hibernation and torpidity.] All terrestrial molluscs hibernate in cold climates, hiding themselves away in the ground between roots and similar sheltered places. In tropical countries some assume a state of torpidity (æstivate) during the hottest and driest season of the year, closing up the aperture of their shells with a temporary lid or door (epiphragm), in order to resist the dryness of the atmosphere. Some of these “summer-sleepers” are endowed with a remarkable tenacity of life. An Australian Pond-Mussel has been known to live a year after being removed from the water; several Land-Snails have revived after a captivity of from two to five years, without any food whatever. One of the most remarkable instances of this kind occurred in the British Museum. A specimen of _Helix desertorum_, a common Desert-Snail from Egypt, was fixed to a tablet in March 1846, and in the same month of the year 1850 it was discovered to be alive. It must have come out of its shell in the interval, and finding it was unable to crawl away, had again retired within it, closing the aperture with a new epiphragm, but leaving traces of slime upon the tablet, which led to its immersion in water and subsequent revival, having passed a period of four years in a dry museum without the smallest particle of food. The actual specimen is here figured, Fig. 1. [Illustration: Fig. 1.[2] _Helix desertorum._ (See black table-case 1.) ] [Sidenote: Economic uses.] The _economic_ uses of molluscs to man are manifold, and will be mentioned in the course of the description of the several families; but here may be the place to direct the attention of visitors to side table-cases B and D at the side of the room, containing some specimens of articles manufactured from shells, such as cameos, flowers, bracelets, brooches, &c. [Sidenote: Geological history.]