528 Bulletin American Museum of Natural History [Vol. LIII 
be compared to that of the Amazon River: between the Narrows and San- 
tarem this is quoted by F. W. Clarke as having a total salinity of 0.059%, 
1.94% of which is Na, 21.12% Ca, and 18.80% SiOz; while at Obidos 
above the mouth of the Tapajos River the total salinity is 0.037%, 
4.24% of which is Na, 14.69% Ca, and 28.59% SiOx. A noteworthy 
feature of these tropical superficial waters is the high proportion of 
silica. In certain of the tributaries of the Orinoco and Amazon, which 
are colored nearly black by organic matter—a condition also met with 
in some of the equatorial affluents of the Congo—the water does not 
contain over 0.016% of mineral matter and lime is practically absent. 
The corrosion so commonly observed on the fresh-water shells, 
especially Melaniidz, Mutelide, and Unionide of the Congo, has not yet 
been satisfactorily explained. Some of the older authors! believed that, 
in some cases at least, it was started by the attacks of other mollusks in 
their craving for lime. This can, however, hardly be the cause of the 
deep corrosion at the beaks of most Unionide and Mutelide, which are 
well hidden within the mud. In their case, Clessin? attributes the corro- 
sion to the chemical action of the soil in which they are embedded, 
although he admits that the periostracum must first be removed in some 
way. He also calls attention to the fact that the shells of certain species 
are much more resistant than others. More likely though, for snails at 
least, the direct action of certain dissolved chemicals, especially of free 
carbon-dioxid, must be incriminated. According to Cooper, Bulimus 
(= Bythinia) and Lymnea that are forced to live in brackish water some- 
times have their summit eaten away.4 It may be observed that in the 
estuaries of the African rivers some of the Melaniide (especially Pachy- 
melania fusca) and Cerithiide often are very heavily decollated, but the 
phenomenon is by no means general. It has also been suggested that the 
corrosion is induced or at least activated by the work of certain micro- 
organisms (confervaceous alge) which are known to live in the calcareous 
1L., Sauley. 1851. ‘Note sur l’Ampullaire cil d’Ammon (Ampullaria effusa Lamarck).’ Journ. de 
Conchyl., IT, pp. 182-140. 
Fischer, P. 1852. ‘Note sur l’érosion du tét chez quelques coquilles fluviatiles univalves.’ Journ. 
De Conchyl., III, pp. 303-310. 
2Clessin, 8. 1871. ‘Die Corrosion der Siisswasser-bivalven.’ Correspondenzbl. Zool. Miner. Ver. 
Regensburg, X XV, pp. 125-130. 
‘See Grier, N. M. 1920. ‘On the erosion and thickness of shells of the fresh-water mussels.’ The 
Nautilus, XXXIV, pp. 15-22. 
Shrubsole,G. W. 1886. ‘On the erosion of certain fresh-water shells.’ Journ. of Conchology, V, 
pp. 66-71. According to this author the shells are corroded in water that contains less than 4 grains of 
lime per gallon and then has a high proportion of carbon-dioxid in solution. 
Moynier de Villepoix, R. 1892. ‘Recherches sur la formation et l’accroissement de la coquille des 
Mollusques.’ Journ. Anat. Physiol. Paris, XXVIII, pp. 461-518, 582-674. 
Cooper, J. E. 1910. ‘Note on decollated shells.’ Journ. of Conchology, XIII, p. 14. 
