1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 145 
Stanleyville, brook near the Falls of the Congo, February, 1915 
- (Lang and Chapin Coll.).! Barumbu (J. Bequaert Coll.). Dolo, in the 
Stanley Pool, one young specimen (Maurice Bequaert Coll.). 
The shell is almost imperforate, thin, semi-transparent olive-buft. 
Summit obtuse, but the first whorl less flattened than in globosa The 
last whorl descends rapidly, causing the penult to appear prominent on 
the left, and the suture to appear deep. The last whorl is rather swollen 
at the shoulder; the outer lip therefore joining the whorl nearly at a 
right angle. ‘The point of greatest excavation of the inner lip is about 
mid-way of the whole right margin of the aperture (while in typical 
P. africana and P. a. globosa this point is far below the middle). The 
columella terminates obliquely and rather abruptly. 
Length, 13.0 mm.; diameter, 9.0 mm.; aperture, 9.3mm.; 444 whorls. 
The size and color vary but little. There is the usual variation in 
degree of descent of the last whorl, though far the greater number agree 
with that shown in Pl. XI, figure 7. Plate XI, figs. 7a, 7b, 7c show 
extreme forms. ‘ 
PI. XI, fig. 7d isan example selected to show fading out of the col- 
umellar truncation. Even in an oblique view in the aperture it is 
weak and strongly oblique. This character, often used in the discrimina- 
tion of species in this genus, varies widely in every large lot we have seen. 
We conclude that its average development in along series is probably as 
reliable as any of the characters of these forms, but its shape in an 
individual specimen has little significance. 
“These tiny shells (Physopsis africana) are so well covered with the 
dirt at the bottom of small brooks or swampy places that they might ‘be 
easily overlooked. They are common about Stanleyville and often are 
found where there is just enough moisture to keep their surroundings 
damp.” . (fi. Li): 
As pointed out by Blacklock and Thompson (1924, Ann. Trop. Med. 
Paras., XVIII, p. 227), Physopsis prefers water that stagnates or runs 
slowly on a muddy bottom, where weed or grass grows in the water, and 
under high or low shade. ° These three factors—mud, weed, and shade— 
appear to be essential for a favorable environment. The snail is not found 
in streams with a clean sandy bottom or with only a thin layer of mud 
over sand. 
1The snail figured by C. C. Chesterman (1923, Ann. Soc. Belge Méd. Trop., ITI, p. 75, Pl., fig. 2) 
from Yakusu near Stanleyville as ‘“‘ Bullinus contortus’’ is Physopsis africana, as shown by specimens 
kindly sent to us by the author. According to Dr. Chesterman it is in that region the probable inter- 
mediate host of what appears to be an intestinal form of Schistosoma hematobiwm (Compare p. 91). 
