1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo O89 
Mysorella, is apparently not closely allied to any of the Ethiopians. 
“Melania” tanganyicensis, unknown to us in nature, does not seem to 
findits natural place among the common fresh-water melanid genera of 
Africa and it might be better placed in a genus of its own, Horea of Bour- 
guignat. The Grandidierix are modified unionids, striking on account 
of the rather small, short, stumpy shell, often with angular contour; 
outside they are covered with a silky periostracum. By far the most 
' interesting type is Pseudospatha, a mutelid with thin, strongly compressed 
valves, polished and bright amber yellow outside, pinkish iridescent 
inside. 
The mollusks of this second group all belong to the so-called littoral 
zone, between shore line and depths of 20 to 50 m. Neothawma is abun- 
dant to depths of 20 m. and it also enters some of the ponds of the coast, 
in the alluvial*plains near the mouth of rivers. Myscrella (?) bisulcata 
was dredged by Stappersin 100 m., but in dead condition, and it probably 
lives near the shore. The Grandidi riz occupy muddy bottoms at depths 
between 25 and 50 m. The thinness of the shell of Pseudospatha, a 
unique feature among Tanganyikan mollusks, must be correlated with 
some peculiarity of its habitat, concerning which unfortunately no in- 
formation is available at present. 
(3) The last group contains the so-called ‘‘thalassoid”’ snails, a 
strange and extremely varied assemblage of forms, so different from 
what we find in the rivers and other lakes of Africa that their relation- 
ship can only be traced through a careful study of the soft parts and 
anatomy and is in some cases still open to question. 
*Syrnolopsis lacustris *Stanleya neritinoides 
4 ; ‘  molirensis *Baizea giraudr 
. e minuta *.. fo eucorapne 
i ‘: “- carinifera Fo Ms. oenauds {f) 
i ie gracilis * 4 gotundata 
*Anceya giraudr * © smithiana 
+ (Ona as ec ne 
+ Seta Pd OPOLare, 
*  “& ° Dusocinae *Reymondia horer 
‘ e ° dig et . es 
*  fereoriferiais i var. giraudt 
; 5 Cs ; 
*Martelia tanganycensis + minor 
; e ; 
*  —— dautzenbergr . tanganyicensis 
*Tanganyicia rufofilosa *Bridouxia giraudt 
si di Ke var. minuta *Spekia zonata 
% 5 fagotiana *Ti phobia horer 
. _ duveyrieriana *Bathanalia howest 
* ns milne-edwardsiana *Timnotrochus thomsoni 
