1897.] MU. W. E. OOLLINGE 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 com- 

 parative 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 geology 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 Jura,ssic age ; 

 and it is by no means improbable that the rift-valley in which 

 Tanganyika now lies began to be formed shortly after these sand- 

 stone beds were raised. 



2. On some European Slugs of the Genus Arion. By 

 Walter E. Collinge, F.Z.S.^ Assistant Lecturer and 

 Demonstrator in Zoology and Comparative Anatomy, 

 Mason College, Birmingham. 



[Eeceived March 2, 1897.] 

 (Plates XXIX.-XXXI.) 



Contents. 



Page 



1. Introduction 439 



2. The Constancy of Anatomical Characters 440 



3. A Eeversion of a Colour Variation 442 



4. The Specific Validity of Arion fuscics, Mull 443 



5. Description of a new Species of ^WoM 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 

 Sluo-s from numerous localities. Some of the results obtained are 

 detailed in the following notes. 



My best thanks are due to Mr. Gr. H. Carpenter, the able editor 

 of the 'Irish Naturalist,' for the very kind manner in which 

 he has helped me to obtain material from Ireland. My 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 Amy Warren, Messrs. A. "W. Borthwick, J. Steele ElUot, 

 W. A. Gain, A. Hartley, H. H. Macnabb,W. Moss, Jas. N. Milne, 



