542 DR. W. A. CUNNINGTON ON THE 



particularly the account given by Edgar Smith (170), who had an 

 unusual opportunity for the systematic study of African fresh- 

 water shells. 



To a lesser degree, the " species-making " of Bourguignat has 

 aiiected the lists of mollusca from the other big lakes of Africa, 

 and in making a selection of forms to be retained and forms to 

 be rejected, I have no doubt acted arbitrarily. Tliere is no 

 need to enlarge upon this point, but while all the lists have 

 been carefully considered with a view to the elimination of 

 synonyms, they must be taken merely as the writer's expression 

 of opinion. Tt is only necessary to emphasise that while finality 

 in such a matter is always impossible, it is especially so in the case 

 of the Mollusca. 



Although the number of genera and species to be included in 

 the fauna of Tanganyika may be a matter of opinion, there are 

 certain striking facts which remain undisputed. Firstly, it is 

 only among the Gasteropods that species described as thalassoid 

 are to be met with ; that is to say, all the Lamellibranchsare non- 

 thalassoid. Secondly, while there are a number of non -thalassoid 

 Gasteropods known to live in the lake, there are more than twice 

 as many which are considered to show a thalassoid aspect. 



The arrangement of the families and genera which has been 

 adopted is, in the main, thfit given by Pelseneer (Treatise on 

 Zoology, Ed. by Lankester, Part V. Mollusca). 



Gasteropoda. 



It has more than once been emphasised that the Gasteropoda, are 

 among the most remarkable of the peculiar animal forins which 

 inhabit Tanganyika. In the adjoining table of distribution, a list 

 is given for each of the lakes under review, of those species which 

 appear to be admissible. This list reaches a total of 133 in all, 

 and it is evidence of the unique position which Tanganyika holds, 

 that 84 of these forms are recorded from that lake, 76 of them 

 being unknown elsewhere. Of these endemic Tanganyikan 

 species, the great majority were described by Bourguignat and 

 others as thalassoid and alternatively by Moore as halolimnic 

 {cf. p. 51?). It has been thought desirable, in view of the special 

 interest attaching to these types, to mark with a * all the genera 

 which have been so designated. Perhaps it is well to explain that 

 none of the species belonging to genera which occur elsewhere 

 have been described as thalassoid by the conchologists, though 

 this does not mean that all the endemic genera are necessarily 

 thalassoid in aspect, there being a notable exception in the case 

 of the ffenus Neothcmma. 



