1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 563 
this fauna are the ponderous clams of the genus Egeria, which are found 
in the mud banks of the main stream from Malela to Katala. In this 
region huge accumulations of dead and bleached valves of Egeria con- 
gica are found on both banks of the stream, often some distance above 
high tide (Pl. LXXT, fig. 2). By Dupont and others they were thought 
to represent ancient alluvial deposits of the stream but, from the junior 
author’s observations, they are kitchen-middens or accumulations of 
valves thrown away by the natives who have for centuries fished these 
clams at the proper season. They usually open them at once upon reach- 
ing the shore, carrying only the animal back to their villages. 
A few miles above Banana the salinity of the creeks is so high that a 
number of truly marine mollusks make their appearance. The most 
striking of these are the peculiar mangrove oyster (Ostrea bicolor Hanley) 
and asmall Mytilus (M. tenuistriatus Dunker). Both these bivalves form 
dense accumulations near the lower end of the prop roots of Rhizophora 
and are, it appears, only fixed on living roots.! The Ostrea is especially 
abundant in the narrower, more muddy creeks and reaches a higher level 
than the Mytilus, being sometimes uncovered by the low tide. Some of 
the natives are extremely fond of mangrove oysters and gather them in 
large quantities. Their taste is generally too muddy to please the 
European palate, but was found that they may be much improved. 
when tied in bags and left in pure sea-water for about a week; if left 
much longer they die. The Mytilus occurs much nearer the sea than the 
Ostrea and the two are not usually found together, at least in abundance. 
It covers only the lowest extremities of the prop roots, being seldom ex- 
posed above low tide, and is often associated with the burrowing isopod 
Sphxroma destructor Richardson.2. Two peculiar marine snails are also 
found with the Ostrea on the mangrove roots. The heavy and extremely 
variable Thats coronata (Lamarck) often covers with its colonies whole 
roots to the exclusion of any other shells. It is always regularly uncovered 
by low tide. The animal is solidly anchored to its substratum by a long 
foot deeply inserted into a crevice. In traveling downstream from 
Malela the increase in salinity of the water is well indicated by the ap- 
pearance of Thais (usually together with Ostrea) and their number 
steadily increases to reach its maximum on the wharf piles along the 
inner shore of Banana Peninsula. 
1The species of Cyrenoida and Congeria are also found among the roots of mangrove, but the ecological 
conditions under which they occur are not known to us, as they were not found alive by Mr. H. Lang 
and the junior author. Their habitat must be quite different from that of the Ostrea and Mytilus. Per- 
haps they seek decidedly less saline water. 
23ee W. G. Van Name, 1920, Bull. American Mus. Nat. Hist., XLIII, pp. 63-67, figs. 16-19. | 
