1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 485 
The South African molluscan fauna is remarkably heterogeneous, in 
part composed of families elsewhere unrepresented in the Ethiopian 
Region. Many of them are well adapted to the peculiar arid or semi- 
arid environment, which may have contributed much to preserve the in- 
dividuality of the fauna. Yet the presence of a large number of peculiar 
genera could by no means be accounted for by present-day ecological 
conditions. The general relationships of the genera may be stated briefly 
as follows. | 
(a) Generally spread Ethiopian groups, including all the Achatinide, 
except Zootecus; the Urocyclide, Streptaxide, and Veronicellide 
(Vaginulide). 
(b) Groups chiefly East African, most of them extending into the 
Oriental Region, and some of the Pupillide into the Palearctic: Zoniti- 
dx, Enid, Pupoides, Microstele, Pupilla, Nesopupa, Truncatellina, 
Lauria, Zootecus, and Tropidophora. | 
(c) Genera most nearly related to those of the Palearctic Region: 
Fauxulus and Austrobalea. As one of us has pointed out, Fauxulus 
appears to beiong close to the European Abzda and Chondrina, and to be 
without near relatives elsewhere. It appears to have been established a 
long time, as there are several strongly marked subgenera.! Austrobalea 
is a recently preposed genus for Balea africana Melvill and Ponsonby. 
This is evidently not a Balea and not closely related to’that genus; but 
its affinities remain uncertain, though evidently with some Palearctic or 
Oriental Clausilide. | 
(d) Families characteristic of the Australian Region: Rhytidide, 
Aperide, and Endodontide. | 
The family Rhytidide, elsewhere Australasian, comprises many 
species of Natalina, a genus which may perhaps require division. The 
_shell-less Aperide are believed to be a much modified branch of the same 
stock, and this family has not been found elsewhere. 
The family Endodontide is widely spread, but the South African 
members (about 70 species of four genera) clearly belong to Austral 
stocks, apparently nearest to those of Australia, Tasmania and New 
Zealand. 
iNearly all South African Pupillide appear to be congeneric with, or clearly related to, species of 
the European middle Tertiary (Oligocene and Miocene) or recent faune. Thus Truncatellina and Lauria 
are merely distinguishable specifically; but both genera occur also in Abyssinia and on mountains of 
Central Africa. Similarly, Pupilla and Microstele are found in European Miocene, in the Oriental 
Region, in the East African Subregion, and in South Africa. Nesopupa 1s now prevalent in Polynesia, 
but it occurs also in the East Indies and India, in the Oligocene and Miocene of Europe, as well as in the 
East African Subregion. We conclude that the wide distribution of Pupillide, not only here, but all 
over the world, is due to the great antiquity of the genera. In Europe many of the recent minor divi- 
sions of genera already existed in the Oligocene. 
? 
