It is well known that the Limnaeidae, and probably most fresh-water Mollusca of sub-temperate regions, can continue to live not merely under, but enveloped in ice, and themselves frozen hard. Garnier relates[35] that, during the winter of 1829–30, some large _Limnaea auricularia_, which had been placed in a small basin, were frozen into a solid mass, experiencing a cold of -2° F. He supposed they were dead, but, to his surprise, when the basin thawed, the Limnaea gradually revived. _Paludina vivipara_ and _Anodonta anatina_ have been known to resist a temperature of 23° F., and the former has produced young shortly after being thawed out of the ice.[36] As far north as Bodø in Norway (67° 37’ N. lat., well within the Arctic circle) there are found no less than fourteen species of terrestrial Mollusca, among them being _Balea perversa_ and _Clausilia rugosa_.[37] _Vitrina_ is one of our most hardy molluscs, and may be observed crawling on bright mornings over the frost-covered leaves of a wood or copse. _V. glacialis_ is said by Charpentier to live in the Alps at a height where the stones are covered with snow from nine to ten months of the year. Many of the _Hyaliniae_ are very hardy. _Arion_, in spite of having no external shell to protect it, is apparently less affected by the cold than _Helix_, and does not commence hibernation till a later period in the autumn. The operculate land Mollusca, in spite of the protection which their operculum may be supposed to afford, are exceedingly sensitive to cold, and the whole group is without doubt a product of tropical or semi-tropical regions (see map at frontispiece). A species of _Helicina_ which inhabits the southern States of North America has been known to be almost exterminated from certain districts by the occurrence of an unusually severe winter. One of the highest altitudes at which a land shell is known to live appears to be the Liti Pass (Himalayas, 14,000 ft.). At this enormous altitude, two species of _Buliminus_ (_arcuatus_ Hutt. and _nivicola_ Bens.) live on juniper bushes among patches of snow. An _Anadenus_ is said to have been found in a similar locality at 15,000 ft., while _Limnaea Hookeri_ has been taken from over 16,400 ft. in Landour. In the Andes of Peru and Bolivia, five species of _Bulimulus_, one of _Pupa_, and one of _Limax_ occur at an elevation of 10,500 to 15,000 ft. Several fresh-water Mollusca inhabit Lake Titicaca, which stands at a height of 12,550 ft. in the Bolivian table-land.