564 Bulletin American Museum of Natural History [Vol. LIIT 
Another common marine snail of the mangrove is a large, turbinate 
Littorina (L. angulifera Lamarck), which, near the mouth of the Congo, is 
usually met with on the branches of mangrove trees above the high tide 
and often also on the leaves. It appears to live most of the time in open 
air and there is even a question as to whether it ever deliberately enters 
the water. Mr. Lang observed that in the central parts of the island of 
Bulabemba, where the mangrove swamps dry out completely during 
July and August, these Littorine secrete at that season a whitish pellicle 
along the édge of attachment between the aperture and the branch, 
which seems to exclude air completely. So solidly are they thus fixed 
that the tree may be shaken without detaching them. In addition, the’ 
outer surface of the shell is thickly coated with crystallized salt, so that 
it shows none of its natural color. These estivating specimens are never 
attached more than three to five feet above the ground, while the snail 
crawls much higher in active life. It is of interest that at that period of the 
year this same species of Littorina is observed crawling about in the creeks 
near Banana. WL. angulifera is also the first of the truly marine forms 
to appear when coming downstream from Malela, much earlier than Thais . 
coronata and Ostrea. The individuals occurring farthest upstream are 
much smaller in size than those living near Banana. The genus Littorina 
is of more than ordinary interest to the ecologist because it contains all 
transitions between marine species that are strictly aquatic and have 
well-developed gills, such as Littorina littorea of Europe, to such as crawl 
at low tide among uncovered sea-weeds (L. obtusata), or occur only at the 
upper limit of high tide (like L. rudis), and finally species that may stay 
alive for months out of water. In the last group belong, not only the 
West African L. angulifera, but several other tropical species: L. muri- 
cata, L. varia, L. fasciata, L. pulchra, and L. irrorata. These changes of 
preferred habitat are accompanied by a gradual reduction of the gills, 
while the wall itself of the mantle cavity becomes more abundantly fur- 
nished with blood vessels, the cavity being filled with air when the animal 
is out of the water. We thus see illustrated one of the ways in which 
pulmonate land snails may have directly evolved from marine forms.! 
The following list includes part of the mollusks which have been 
observed in brackish water at the very mouth of the Congo, that is, 
near Banana on the right shore and near San Antonio on the left bank. 
To be complete, it should include many true marine forms which live 
Wee V. Willem, 1895, Bull. Ac. Belgique, Cl. des Sci., (3) XXIX, pp. 73-88; P. Pelseneer, 1895 
ee i Biologie, XIV, 2, pp. 351-393, Pls. xrv-xvir1. The genus Littorina is here used in the older 
nd wide sense. 
