482 Bulletin American Museum of Natural History [Vol. LI 
The dividing line between these two subregions as drawn on the 
map by a heavy black line, is of considerable biogeographic interest. A 
comparison with an orographic map of Africa (Map 11) shows that this 
line nowhere coincides with mountainous ridges which could act as barriers 
tothemigration of plantsandanimals. Quitetothe contrary, the boundary 
of the West African subregion frequently cuts across the divides of the 
great river systems and, where it follows such hydrographic boundaries, 
these are, asa rule,so low and uniform as to be inconspicuous. The present 
limits of the two subregions of the Ethiopian Region are entirely due to 
present and past climatic conditions. As Chapin expresses it, the ‘“‘ West- 
ern fauna owes its distinctness to ecological or climatic conditions, as 
comparison with the rainfall map of Africa (Map 8) will prove. It is the 
principal area of more than 60 inches annual rainfall; and I am led to 
believe—from its dependence on the most fundamental movements in the 
atmosphere—that such a humid area near the equator is of vast 
antiquity.” 
It is, of course, not to be expected that highly mobile animals of 
relatively recent evolution, such as birds and mammals, will always 
agree in the distribution of families and genera with the relatively seden- 
tary and more ancient mollusks. Yet the limits of the two avifaunal 
main divisions of equatorial Africa proper agree fairly well with those 
based on the distribution’ of land mollusks. South of the Zambezi, 
however, the avifaunal districts proposed by Chapin no longer serve our 
purpose. The South African Subregion (Map 12) is a division as least as 
strongly marked as the West African, from a malacological standpoint. 
Its limits do not appear to coincide with those of Sharpe’s South African 
Subregion,’ nor with Chapin’s Southeast Veldt District and Southwest 
Arid District. : 
1. Wustr AFRICAN SuBREGION.—The following genera of terrestrial 
mollusks are confined to this part of the Ethiopian Region: 
Achatinidee Streptaxide 
Archachatina Artemonopsis 
Columna Zonitidee 
Pseudachatina Trochozonites 
Callistoplepa Thomeonanina 
Atopocochlis Urocyclida? 
Pseudotrochus Dendrolimax 
Perideriopsis Aspidelus 
Ceras . Thryophorellide 
Thomea . ~ Thryophorella 
Pyrgina 
11893, Natural Science, III, chart facing p. 108. 
“Various other genera of Urocyclide and Helixarionine doubtless remain to b 
of these groups is still very little known. eg ee Me range 
