So~tas— On the Origin of Freshwater Faunas. 109 
Dreissena subglobosa (Partsch. ) P. loxostomia (Sandb.) 
D. triangularis (Partsch. ) P. achatinoides (Desh. ) 
D. subcarinata (Desh. ) P. duboisi (C. Mayer). 
D. simplex (B. de Marny.) Melantho sadleri (Partsch.) 
D. schrockingeri (Fuchs. ) | 'Tulotoma zelebori (Hornes). 
Didacna conjungens (Partsch. ) _T. rudis (Neumayer). 
Unio vukotinovici (Hérnes. ) _T. avellana (Neumayer). 
U. flabellatus (Gdt.) Bithynia verneulii (C. Mayer). 
Pisidiun priscum (Eich.) iB. tentaculata (Linn.) 
Neritina, sp. _ Valvata balatonica (Rolle). 
Melanopsis martiniana (Férussac) | V. piscinalis (Miill. ) 
M. bouei (Férussac) | Planorbis varians (Fuchs. ) 
Melania escheri (Brgn. ) Carinifer quadrangularis 
Goniochilus costulatum = (Fuchs.) (Neumayer). 
Turbonella inaspecta (Fuchs. ) | Limnzeus velutinus (Desh.) 
Paludina vukotinovici (Frauenfeld) | Valenciennia annulata (Rousseau). 
It will be seen that the Melaniina of the Congerien beds have disappeared from the 
Aralo-Caspian area, as indeed they have from all the northern Palearctic zone, but 
Adacna and Dreissena are common in both the Caspian and Congerien faunze. One 
species of the last-named genus (Dreissena simplex) is of particular interest, for it 
is stated by 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. 
unguiculus of the Headon Hill strata (Upper Eocene). Since Dreissena Brardii 
(var. caspius) also occurs in the Caspian according to Grimm, we have in this 
species a series of forms which, commencing with D. wnguiculus in the Upper 
Eocene, has persisted with scarcely more than varietal modification down to 
existing times. 
If the genus Dreissena originated in Europe in Kocene 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 Nyassa, which are best known, 
are given in the following list, compiled from Mr. Edgar Smith’s descriptions.* 
Some of these forms have a very marine aspect, particularly Neothauma, 
Tiphobia, Paramelania, Limnotrochus, and Syrnolopsis, 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 
* Proceedings, Zool. Soc. 1880, p. 884; 1881, p. 276 and p. 658. 
