1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo o91 
Crosse gave the name of Tanganyicia.”’ Other snails, such as the 
Syrnolopside (Syrnolopsis and Anceya), Limnotrochus thomsoni, and 
Chytra kirkit, live freely on the muddy or gravelly bottom, overgrown 
with aquatic weeds, of the littoral zone, between the shore and depths of 
50 to 80 meters. Here they are accompanied by the snails and mussels 
of our second group, which are much more typical of this particular 
habitat than any of the thalassoid species. Finally, a relatively small 
number of types inhabit the lake bottom at greater depth, between 50 
and 200 meters. Among these the most frequent is T7phobia hore, which 
Moore found in 100 fathoms and over, while Stappers dredged it in depths 
of 90 to 160 meters. Moore also writes that Paramelania (Bythoceras) 
wridescens and P. (B.) mincr were obtained between 300 and 1,000 feet. 
Bathanalia howesi also is a true deep water form: Stappers found it in 
76 meters. Beyond a depth of 200 meters there is very little if any mol- 
luscan life, in what should properly be regarded as the true ‘‘ bathylim- 
netic’’ zone, although it would seem that these deeper regions have not. 
yet been adequately investigated.! 
Most thalassoid snails present a number of striking peculiarities 
by means of which they differ markedly from the usual fresh-water types. 
As they belong to many different species, several of which are of large 
size and extremely abundant, they form an assemblage not to be dupli- 
cated anywhere else. With very few exceptions the shells are solid, thick, 
and heavy, which points to a steady and fairly abundant supply of lime. 
In addition they show none of the corrosion at the top of the spire which 
is so common a feature of other African fresh-water gastropods, and their 
surface is generally free of incrustations or coating of foreign matter. 
As in many truly marine snails, the periostracum is comparatively 
thin and the outer surface is clean and often smooth. They are handsome 
shells, beautifully sculptured and often provided with spines or protuber- 
ances. The size, the proportions of the spire, the shape of the aperture, 
and the details of sculpture are as a rule so variable that the limits 
between the species are hard to draw. The strangest among the thalas- 
soid genera are Tiphobia, Bathanalia, Chytra, Limnotrochus, and Spekia, 
in which it would be impossible to recognize Melaniidz without a study of 
the soft parts, opercula, and radule. 
In our opinion the term ‘‘thalassoid” applied to these mollusks has 
befogged the whole “Tanganyika Problem.” Even authors like Dr. 
Cunnington, who discredit the relationships of these genera with marine 
10ther deep fresh-water lakes, in Switzerland, Japan, and elsewhere, possess in their bathylimnetic 
zone a number of small, usually thin-shelled, fragile snails and mussels, evidently derived from 
normal species of the littoral zone. No such types have thus far been found in Tanganyika. 
