A clue to climatic conditions is frequently furnished by the physical characters of accumulations, especially terrestrial ones. The accumulations containing a large percentage of hydrocarbons have probably been formed under fairly temperate and moist climatic conditions, whilst the existence of millet-seed sandstones associated with chemical deposits points to desert conditions and inland lakes, requiring a dry climate and probably a warm one. Glaciated surfaces and glacial deposits of course indicate a low temperature. Some geologists profess that occasionally they can even determine the direction of the prevailing winds during past periods, by examination of the character of ripple-marks, rain-pits and other features, though it is doubtful whether much reliance can be placed upon these obscure indications. Useful as is the physical evidence supplied by deposits, as an index to the conditions under which they were formed, it is usually only supplementary to the evidence derived from a study of the fossils. Fossils when present in the rocks, usually supply considerable information concerning the prevalent conditions during the deposition of the rocks. By them we can not only separate marine from terrestrial deposits, but also freshwater deposits from æolian accumulations; each kind of deposit will generally contain the remains of organisms which existed under the conditions prevalent in the area of formation of the rock, though it is of course a frequent thing for a terrestrial creature or plant to be washed into a freshwater area or into the sea. In an æolian deposit, the invertebrate remains may be those of any air-breathing forms, as insects, galley-worms, spiders, scorpions and molluscs. The land-molluscs are all univalve. Of vertebrates, we may find the bones and teeth of amphibians, reptiles, birds and mammals. Occasionally freshwater or even marine forms may be found in an æolian deposit, but they will be exceptional. Marine shells are often blown amongst the sand-grains of the coastal dunes, and seagulls and other birds frequently carry marine organisms far inland. The creatures frequenting fresh water differ from those of the land and of the sea. The most abundant vertebrate remains will be those of fishes, and of the invertebrates we find mollusca preponderate. The variety of molluscs is not so great as in the case of marine faunas. The bivalves always possess two muscular scars on each valve (except adult _Mulleria_); whilst many marine shells as the oyster have only one muscular scar on each valve. (See Fig. 11.) [Illustration: Fig. 11. _A._ Monomyary shell with one scar. _B._ Dimyary shell with two scars.]