566 Bulletin American Museum of Natural History [Vol. LIII 
colored mangrove crabs of the genus Sesarma and the peculiar, amphibious 
fish Periophthalmus kelreutertx. An environment typical of P. fusca is 
shown on Pl. LX XV, representing the upper end of a creek near Kunga 
at high and at low tide. Pl. LX XVI, fig. 1, a detail study of one of the 
mud flats seen in Pl. LX XV, illustrates the manner in which the snails are 
partly buried in the slimy, algee-covered ooze. In such surroundings the 
shells are thickly coated with dirt and an adherent layer of hydroxide of 
iron, similar to that which encrusts the Potadoma in the brooks of the 
Upper Congo and Ituri forests. This crust is also produced by iron- 
depositing bacteria, which in brackish water appear to belong to the 
same species as in fresh-water.! 
Pachymelania fusca also occurs in a much smaller, evidently depaup- 
erate form, on the sea shore of Pt. Padrao, west of San Antonio, together 
with Neritina glabrata, Lanistes intortus, and Pachymelania aurita, all of 
these snails being here quite clean of dirt, although frequently covered 
with barnacles. The smaller size of P. fusca at Pt. Padrao is evidently 
due to the higher salinity of the environment. In this connection it may 
also be pointed out that Neritina oweniana in brackish water at Malela 
reaches only half the size of the species in the practically fresh-water at 
Zambi.? Pens 
All these mangrove mollusks have decidedly the habit of congregat- 
ing in colonies. They may be found by the thousands in certain spots, 
while in nearby locations under seemingly similar conditions they are 
totally absent. Upon arriving at Banana, for instance, one at first looks 
in vain for living examples of Pachymelania and Potamides. Those one 
finds most commonly are dead specimens, evidently washed ashore or 
usually inhabited by hermit crabs, as shown in Pl. LX X XVI, fig. 2. 
The distribution of mollusks in the estuary of the Congo as sketched 
above is evidently regulated by the variations of salinity. Unfortunately 
we were unable to find that analyses of water have been made at 
any point in the lower course of the river, so that we must content our- 
selves with the general statement that salinity increases upon nearing 
the ocean from the interior. The variations are probably best shown by 
the concurrent changes in the vegetation, and the zone covered by man- 
grove forest (ehizophora) may be accepted as that of decidedly brackish 
water. The problem is, however, by no means the simple matter of 
1See North, L. and Bridenstine, I. J. 1922. ‘Some notes on iron-d iting bacteria.’ i 
Geol, XVI ee ron-depositing bacteria.’ Economic 
2Similar variation in size apparently due to changes in the salinity of the water were noted b 
M. ‘Metcalf (1904, American Naturalist, XX XVIII, pp. 565-569, Pl. ) for the West Indian Woniied 
virginea. 
& 
