520 Bulletin American Museum of Natural History [Vol. LIT 
of the Great Lakes), and, if we add some that are more or less dubious 
on account of possible misidentification or synonymy, we reach a total 
of 160, representing 33 genera. These figures are very low indeed, con- 
sidering the size of the territory (about 900,000 square miles, a little less 
than one-third of the area of the United States), and the abundance of 
superficial water (the Congo system possesses over 5,000 kilometers of 
commercially navigable water-ways alone!). Naturalists not personally 
acquainted with African conditions will probably assume that this 
poverty is only apparent and merely reflects insufficient investigation. 
Our studies of the African fluviatile mollusks have, however, brought us 
to a different conclusion. No doubt many species remain to be discovered 
in the Congo basin, as elsewhere on the continent. Yet it should be 
remembered that collections have been made at present in many different 
localities, in some cases by professional zodlogists. It is hard to believe 
that all these collectors should have overlooked a large proportion of the 
aquatic mollusks in the localities they visited, especially since these 
animals are, with few exceptions, of fair or large size, and not minute 
like so many of the terrestrial species.1. The extensive material gathered 
by the American Museum Congo Expedition, which numbers many 
thousands of specimens, as well as the junior author’s personal experience, 
fully justify our opinion that the African fresh-waters actually nourish 
but few types of mollusks.?, There is nothing here comparable with the 
abundant and varied mollusk faunz of many of the North American 
river systems. 
7 The Ecological Factors 
Since aquatic mollusks normally spend most of their existence im- 
mersed in a liquid medium, they are more or less independent of some of 
the environmental factors that influence terrestrial snails and slugs, 
such as vegetation, topography, and climate. The physical and chemical 
It is of course among the smaller forms, such’as'Ancylide, Amnicolide, and Spheriide, thatinter- 
vee accessions may most readily be made and these are therefore especially recommended to future 
collectors. 
*Roubaud has quite expressed the feelings of the disappointed collector in a letter written from 
Brazzaville to Germain, which we beg to quote as fully pertinent to our subject: ‘“‘Désle début de mon 
arrivée, j’ai visité les bords du fleuve et les jles, m’attendant A trouver des monceaux de coquilles en 
cordon littoral. Je n’ai rien rencontré, sauf, de loin en loin, et le plus souvent vivants, des individus 
isolés que les eaux venaient d’abandonner. J’ai parcouru déjA pas mal de chemin dans le Congo de 
l’Hinterland, j’ai fait, tant dans la vallée de l’Alima que sur les plateaux de la route des caravanes, des 
recherches nombreuses sur un parcours de plus de 1,200 kilométres, autant que me le permettaient les 
loisirs des étapes, et si j’ai été surpris d’une chose, c’est de la grande rareté des Mollusques au Congo. 
La faune du Pool |Stanley Pool! méme, qui est la plus riche, me parait jusqu’A présent singuliérement 
pauvre, étant donné sa surface qui couvre en largeur prés de 40 kilométres.”’ * (L. Germain, 1909, 
Arch. Zool. Expér. Gén., XLI, p. 109, footnote.) 
‘For instance, no less than 60 species and subspecies of snails and mussels have been found in the 
the Big Vermilion River system, which drains about 1,500 square miles in Illinois. See Baker, F. C. 
io ‘The molluscan fauna of the Big Vermilion River, Illinois.’ Ilinois Biol. Monogr., VII, 2, 126 pp., 
8. 
