Observations on the Distribution of Land and Freshwater Mollusca in the Southern Libyan Desert

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Abstract
In the course of a reconnaissance of the southern Libyan Desert and north-western frontiers of the Anglo-Egyptian Sudan, led by Major R. A. Bagnold (1933), I searched carefully for shells in oases and other localities. Of species now living in other parts of Africa, some were found only in the geological deposits of some oases, while a few appear to be living in this region in certain circumstances. Thus, shells of a land-snail common in tropical Africa were found in regions now totally arid, and the molluscs had evidently lived and multiplied there recently. Moreover, the freshwater species found in widely separated localities can breed only near or in water, and their dispersal or survival is an interesting problem. Evidence will be given to show that small and purely temporary changes of precipitation—a matter of good or bad rain years—have disproportionate results in arid lands. Rapid re-colonization by molluscs follows exceptional rains, and in some instances a few individuals have survived long droughts. An explanation is offered in this paper of the alternations of barren and highly fossiliferous continental deposits found in this and other arid regions. Deductions as to climatic changes, made from the evidence of such alternations of superficial deposits in lands now or previously arid, are unsafe, and may give rise to confusion or doubtful correlations. This paper, written in 1934, was intended to be a study of the past and present conditions of the southern part of the Libyan Desert, and to demonstrate some of the

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