464 Bulletin American Museum of Natural History [Vol. LII] 
animals.! Mollusks need considerable quantities of lime for the building 
up of their shells and where limestone is conspicuously absent, as is the 
case over very large areas of the Belgian Congo, this side of their meta- 
bolism must be seriously handicapped. Many of the land shells of the 
Congo are thin or transparent, and frequently of minute size. Others, 
such as certain of the mountain Helicide and the numerous Helixarioni- 
dee, have soft or corneous shells. The scarcity of lime has undoubtedly 
favored the development of Helixarionidz and of slugs of the families 
Vaginulidze and Urocyclide. The Achatinine are somewhat of an 
exception to the rule, but even in their case it may be observed that the 
shells of these vegetarian snails do not attain in the Congo basin the 
large dimensions and weight of some of their South and East African rela- 
tives. There is also a conspicuous difference in size and thickness of the 
shell and in number of individuals between the Achatine of the rain 
forest, where the soil is better protected and therefore less leached, 
much of the mineral matter being kept in solution in the groundwater ; 
and those of the savanna, where the soil is more thoroughly deprived 
of soluble matter and frequently consists of bare granite, gneiss or 
quartz, bleached sand or laterite. The acidity or alkalinity of the soil 
is also a factor of some importance in the distribution of land snails and 
may often account for the local occurrence of certain species. Atkins 
and Lebour found in Ireland that snails with hyaline shells may occur 
on soils showing a wide range of acidity, while those with calcareous 
shells are restricted to more alkaline situations. Granite and quartzitic 
regions possess few species, basaltic districts are decidedly richer, and in 
limestone areas both species and individuals are very numerous. 
In his discussion of the causes which might be responsible for the 
poverty of the molluscan fauna of the Congo, E. Dupont® dwells upon 
the almost total absence of lime-containing rocks over much of the Lower 
and Middle Congo and he furthermore calls attention to the abundance 
of organic matter in the flowing waters. Yet he does not believe that 
either of these two factors is to be blamed, for, he says, certain Congo 
mollusks are nevertheless able to secrete rather heavy shells. The 
a a Re eee Be eo ae Ee 
1See Strobel, P. 1876. ‘Saggio sui rapporti esistenti fra la nat del ] la distri i i 
Meyuege val feet es Coe Atti Soe. Ital. Sci. Nat., XIX, a 19-42. ans dei 
essin, 8. . ‘Ueber den Einfluss kalkarmen Bod { di dj : - 
pondensi Zo, Miner. Ver Regensburg xeven s ig ogee auf die Gehausenschnecken.’ Corres 
oldhaus, K. . ‘Ueber die Abhingigkeit der F ben 
Zool, Kenge Gray (816), oe gigkeit der Fauna vom Gestein.’ Verh. VIII. Internat. 
ins, W. R. G. and Lebour, M. V. 19238. ‘The hydrogen ion concentrati {th il 
ae oes ee in relation to the distribution of snails.’ Seientit . Proc. Roy. Dich coe. N.S, VIL. 
’Dupont, E. 1891. ‘Sur des Mollusques vivants et postpliocé illi ; 
Congo en,1887.’ Bull. Ac. Sci. Belgique, (3) XX, (1890), pp. 550-566, ee voyage au 
