1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 461 
in 19° N., 20° 30’ N., and 21° N. are very few in number considering the 
extension of the territory involved and especially compared with the 
numerous records from south of the sixteenth northern parallel. Nor is it 
unequivocally stated that any of the specimens from the arid area of the 
Sudan were found alive. Dead specimens may well be subfossil remains 
of former flourishing colonies of these snails. Moreover, the possibility 
of their having been transported by native caravans, either dead or 
alive, as amulets or utensils, or accidentally with food or merchandise, 
cannot be entirely discarded.! : 3 
The ecological and, to some extent also, the historical causes respon- 
sible for the present composition and distribution of the mollusk fauna 
are in many respects so different for the terrestrial and the aquatic 
species, that they are most conveniently studied separately for each of 
these groups. 
EcoLoGy AND DISTRIBUTION OF TERRESTRIAL MOLLUSKS 
The Ecological Factors 
The main ecological factors which affect the distribution of land 
mollusks may be brought together under the headings of climate, vege- 
tation, soil, topography, and other organisms.’ 
With few local exceptions due to altitude, the climate of the Belgian 
Congo is everywhere of the equatorial or tropical type. Most of the 
territory lies within the belt of calms or variable winds, the doldrum belt 
or equatorial cloud-ring, as it is sometimes called by meteorologists. 
When the sun is in the south (from December to March), the dry north- 
eastern trades blow over the savanna district of the upper Ubangi and 
upper Uele. Again, in Upper Katanga and southern Kasai, the south- 
eastern trades make themselves felt during the southern dry season 
(from May to September). The temperature is very uniform near the 
equator and, except at the top of some of the high plateaus and mountain 
ranges, it never drops to the freezing point, so that its variations are- of 
little if any import for life. The total amount of rainfall is considerable® 
1The discrepancy between Germain’s eastern limit of Limicolaria in Somaliland and that given on 
our map appears to be merely due to his transferring longitudes calculated on the Greenwich meridian 
without reduction to a map based on the Paris meridian. With the exception of Limicolaria oviformis 
Ancey, from the ‘northern coast of Somaliland,’’ a species which we had eliminated as of doubtful 
provenance (the specimen was at first said to have come from Aden!), the same locality records were 
used by both Gérmain and ourselves. . : 
2Of the older accounts of the influence of the environment upon mollusks the most interesting per- 
haps are those of Locard, A., 1881, ‘Etudes sur les variations malacologiques. II,’ Ann. Soe. Agric, 
Lyon, (5) III, pp. 189-748; 1893, ‘De l’influence des milieux sur le développement des Mollusques, 
op. cit., (6) V, (1891), pp. 1-140. A recent, systematized and exhaustive review of the subject is that by 
Pelseneer, P., 1920, ‘Les variations et leur. hérédité chez les Mollusques, Mém. in-8° Ac. Belgique, Cl. 
des Sci.,.(2) V, pp. 1-826 (the influence of external factors is fully discussed on pp. 476-602). 
