SOLLAS On the Origin of Fn./<i<'<i/<r Faunas. 



Dreissena subglobosa (Partseh.) 



D. triangiihiris ( I'artsrh.) 



D. subearinata (Deh.) 



D. simplex (B. de Miirnv.) 



D. schrockinireri (Furhs. ) 



Didaeua conjuugens (Partaoh.) 



Unio vuki'tinovici ( ll'">ri! 



U. HaMlatus (Gdt.) 



I'isidiun priscum (Eirh.) 

 Neritina, sp. 



MelanopMs inartiuiana (Fdrussac) 



M. bouei (Fdrussac) 



Melania eseheri (Brgn.) 



( loiiioehilus costulatuni (Fuchs.) 



Turbonella innspecta (Fuchs.) 

 Paludina vukotinovici (Fraucnfeld) 



P. loxostomia (Sandb.) 



P. achatinoides (Desh.) 



P. duboi>i (C. Mayer). 



M< lautho sadleri (Partsrh.) 



Tulotoma zelebori (Homes). 



'l\ nulls (Neumayor). 



T. avellana (Noumayer). 



Bithynia verneulii (C. Mayer). 

 B. tentaculata (Linn.) 



Valvata balatonica (Rolle). 



V. pi.scin.ilis (Miill.) 



Planorbis varians (Fuchs.) 



Carinifer quadrangularis 



(Neumayor). 



Limnseus velutinus (Desh.) 



Valenciennia annulata (Rousseau). 



It will be seen that the Molaniina of the Congerien beds have disappeared from the 

 Aralo ( 'aspian area, as indeed they have from all the northern Palaearctic zone, but 

 Adacna and Dreissena are common in both the Caspian and Congerien fauna?. One 

 species of the last-named genus (Dreusena simplex) is of particular interest, for it 

 is stated hy Sandberger to be closely allied to D. Brardii, a species which occurs 

 in Lower Miocene deposits, and which is preceded by a very similar form, D. 

 iiiiffuiculuK of the Headon Hill strata (Upper Eocene). Since Dreissena Brardii 

 (var. caspins) also occurs in the Caspian according to Grimm, we have in this 

 a series of forms which, commencing with D. unguiculus in the Upper 



ene, has persisted with scarcely more than varietal modification down to 

 existing times. 



If the genus Dreissena originated in Europe in Eocene times, it has acquired 

 an extensive distribution since, for it is now found living in Eastern Europe, 

 Asia, Africa, and America. 



In the lakes of Central Africa, which have not experienced the severe climate 

 and other trying conditions of the northern lakes, we find a remarkable assemblage 

 of freshwater Mollusca. Those of Tanganyka and Xyassa, which are best known, 

 are given in the following list, compiled from Mr. Kdgar Smith's descriptions.* 



- ;iie of these forms have a very marine aspect, particularly Neothauma, 

 Tijihobia, Paramelania, Limnotrochus, and Syrnolopsu, so that at first sight one 

 might be disposed to regard them as of comparatively very recent origin : further 

 consideration, however, will render this view improbable. On the next page the 

 molluscan genera of Lake Tanganyka are assigned to their respective families, and 



Procetdinyt, Zool. Soc. 1880, p. 884 ; 1881 , p. 276 and p. 658. 



