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Show 1897.] MR. W. E. COLLINGE ON EUROPEAN SLUGS. 439 to point to the general conclusions that Tanganyika is unlike most of its sister lakes in having been contaminated by incursions of marine organisms at a rather remote period of time. It is certain that there are two distinct faunas in Tanganyika, but the ultimate determination of the nature of the more peculiar series which I have just described can only be attained by a thorough comparative study of the morphology of the selected specimens which I have brought home. In the meantime, however, it may not be unprofitable to point out that there is nothing in the geoiogy of the Tanganyika district which precludes the likelihood of that part of Africa having been occupied by an arm of the sea in ancient times. The massive sandstone formations of this district are probably of Jurassic age; and it is by no means improbable that the rift-valley in which Tanganyika now lies began to be formed shortly after these sandstone beds were raised. 2. On some European Slugs of the Genus Avion. By W A L T E R E. C O L L I N G E , F.Z.S., Assistant Lecturer and Demonstrator in Zoology and Comparative Anatomy, Mason College, Birmingham. [Received March 2, 1897.] (Plates XXIX.-XXXI.) CONTENTS. Page 1. Introduction. 439 2. The Constancy of Anatomical Characters .. 440 3. A Reversion of a Colour Variation 442 4. The Specific Validity of Arion fuscus, Miill 443 5. Description of a new Species ot Arion 444 6. Synopsis and Classification of the Genus 445 7. Bibliography 449 Explanation of the Plates 450 1. Introduction. During the past five or six years I have been enabled, through the kindness of numerous malacologists and other naturalists, to examine exceedingly large series of the various species of European Slugs from numerous localities. Some of the results obtained are detailed in the following notes. M y best thanks are due to Mr. G. H . Carpenter, the able editor of the 'Irish Naturalist,' for the very kind manner in which he has helped m e to obtain material from Ireland. M y thanks are also due to the following ladies and gentlemen for material they have from time to time sent to me:-Misses M . J. Delap and A m y Warren, Messrs. A. W . Borthwick, J. Steele Elliot, W . A. Gain, A. Hartley, H . H . Macnabb,W. Moss, Jas. N . Milne, |