1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 377 
Fig. 85. Cprenoida rosea brevdentata Pilsbry and Bequaert. Type Kunga. 
PRIONODESMACEA 
Unionidse 
Fresh-water bivalves of moderate or large size, the shell-covered with a strong 
epidermis, having an external ligament; the interior conspicuously pearly; hinge 
provided with cardinal and posterior lateral teeth, with cardinals only, or without 
teeth; pallial line not sinuate. 
Foot large and muscular, without byssus in the adult stage; the two adductor. 
muscles subequal; mantle margins usually united to form short branchial, anal, and 
generally supra-anal siphons or orifices. Gills or parts of them modified to form 
marsupia n which the eggs develop into a larval, bivalve stage known as the glochi- 
dium. Dicecious, often with sexual differences in the shell, but these have not been 
studied in the African forms. | 
The family is of world-wide distribution and great antiquity, being 
known as early as the Triassic. Although Unionids are very common 
on the African continent, they appear to be poorly represented in Mada- 
gascar. But four species (Celatura carver Germain, C. geayi Germain, 
Unio madagascariensis Sganzin, and U. malgachensis Germain) have 
been recorded thus far from that island. None are known from the 
Mascarenes. 
For practical purposes, the Ethiopian Unionide and Mutelide are 
distinguished by the possession of lamellar lateral teeth and shorter 
pseudocardinals by the former family. This dentition is not found in 
any Ethiopian Mutelide, so far as we know. 
The species of Unionidee known from the Belgian Congo have been 
here provisionally arranged in the two genera Celatura and Parreysia, 
Lzvirostris Simpson being treated as a subgenus of Czlatura. Some of the 
South African species, however, appear to be true Unios and Simpson 
