1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 453 
triangular in section, without spines, its shape probably due in large part 
to the situation. 
The specimen from Kasenga (Pl. XXX, figs. 2-2a) isa young individ- — 
ual of the var. cailaud:. It differs from typical ellzptzca by the more 
elongate beaks. | 
‘The river oysters (Htheria elliptica), though occurring throughout 
the entire extent of the Congo basin, were only found on certain rocky 
sections about rapids and falls of fair-sized streams, where they form 
‘banks’ in stony and generally turbulent places, often ten feet and more 
below the surface of the rushing floods. Even when the rivers are at 
their lowest level, as happens during the dry seasons, the uppermost 
shells alone become exposed. The mollusks attach themselves by one 
valve to the rock and also to other shells of their kind. In some places 
they are three and four deep and sometimes so hemmed in that probably 
not a few are thus suffocated, as many shells close to the rock base con- 
tain no animal. In spite of their great size (5 to 8 inches), generally 
spinous appearance, and relative abundance, they easily escape the 
attention of travellers, hidden as they are by the brownish torrents. 
Furthermore, like the stones about them, they are covered with a mass of 
fluviatile mosslike animals (Bryozoa and Spongillide). A number of 
such forms found in these Htheria associations have been recorded in 
various localities in Africa, such as fresh-water Bryozoa of the genus 
Plumatella'; Spongilla sumatrana from the Nile and Rukagura River, 
Usegua, Tanganyika Territory, Corvospongilla loricata from an unknown 
locality, C. micramphidiscoides from rapids of the Aruwimi River near 
Banalia at 3 to 4 m. depth, where Schubotz collected also Spongzlla 
(Stratospongilla) schubotei.2  Annandale® describes two other species 
from the Nile—Spongilla (Eunapius) extherie and Cervospongilla sca- 
brispiculis. As Dr. Bequaert and others‘have pointed out, a more care- 
ful investigation will prove highly interesting, especially as regards other 
mollusks that find a home in such favorable places as the roughened, 
corrugated surfaces of these fresh-water oysters. Among them are small 
clams (Spheeriide), several forms of which are now known to occur in 
the Congo basin on shells of Etherza elliptica, such as Kupera mediafricana 
in the Tshopo River near Stanleyville, H. m. ethervarum in the Dungu . 
River near Faradje, E. bequaerti in the Luapula River at Kasenga, and 
E. sturanyi in the Lower Congo at the District of Cataracts. There are 
EET eee ee 
1Ulmer, 1913, in ‘Wiss. Ergeb. D. Z. Afr. Exp. 1907-1908,’ IV, Zoologie, 2, p. 286. 
2Weltner, 1913, loc. cit., pp. 476-481. 
31913, Rec. Indian Mus., IX, pp. 237-238. 
