1927]. Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo pee 
matter of living shells.1 -The problem is evidently rather complicated and 
will require further investigation for a satisfactory solution. 
Fresh-water snails and mussels of stagnant or slow-moving water in 
the Belgian Congo are usually covered with a thick and very adherent 
coat of brownish substance, consisting mainly of hydroxide of iron. It 
appears to be deposited by the action of several bacteria belonging to the 
genera Cladothriz, Spirophyllum, Siderocapsa, Leptothrix, and Crenothriz, 
which are also responsible for the production of brown flocks or strings in 
water. These bacteria absorb ferrous carbonate in solution in the water | 
and redeposit the iron later as ferric hydroxide. Other species of Creno- 
thrix precipitate pure white to yellowish white oxid of aluminum or 
brown to almost black oxid of manganese. These bacteria “occur chiefly 
in ground waters and only grow with rapidity when the dissolved oxygen 
is lacking, or nearly so, and when the special salts are present which they 
precipitate. The presence of much organic matter seems to favor the 
growth, but the two former conditions are absolutely necessary. The 
absence of light and the presence of carbonic acid in the water are also 
usual conditions and seem to favor growth.’ 
When there is a plethora of caleareous matter in the water the shells 
may become covered with a heavy deposit of limy matter, often in the 
form of tufa. This is sometimes found on the exposed end of Unionide 
that live partly buried in the bed of rivers or ponds, but is seldom the 
case in the Belgian Congo. The free ends of the Tanganyikan [ridina 
spekit are usually encrusted with a growth of alge in which grit is em- 
bedded. Alge of various. kinds, as well as fresh-water sponges, often 
overgrow the fluviatile mollusks and completely mask the true colors and 
minute, characteristic sculpture. In order to observe these it is necessary 
to clean them with a weak acid, preferably with a saturated solution of 
oxalic acid. The shells of Planorbis are sometimes coated all over with 
diatoms. 3 
We now come to a brief examination of the ecological relations of 
fresh-water mollusks to other organisms. These animals feed, as a rule, 
upon vegetable matter, mainly microscopic forms of life, such as diatoms, 
er ee 
Noll, F. C. 1882. ‘Micrococcus conchivorus (Vorlaufige Mitheilung).’ Zoolog. Garten, XXIII, 
pp. 156-159. Attributes corrosion to the attacks of a unicellular alga, Micrococcus conchivorus. 
Wedl, C. 1859. ‘Ueber die Bedeutung der in den Schalen von manchen Acephalen und Gasteropo- 
den vorkommenden Canale.’ Sitz. Ber. Ak. Wiss. Wien, Math. Naturw. Kl., XX XIII, (1858), pp. 
451-472, Pls. 1-111. 
a es - ml Flahault, C. 1889. ‘Sur quelques plantes vivant dans le test calcaire des Mol- 
lusques.’ Bull. Soc. Bot. France, XXXVI, pp. exlvii-clxxvi. | 
2An excellent account of the iron depositing micro-organisms 18 that of Ellis, D. 1919. ‘Iron Bac- 
ia.’ (London), xix-+179 pp., 5 Pls. 
teria : ae si New species of Crenothria (C. manganifera).’ Trans. Amer. Mier. Soc., 
XXIII, pp. 31-39, Pl. x. 
