1927] | Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 247 
Mangrove swamps of the estuary of the Congo River: Malela; 
Kunga; Bulabemba Island; Banana; also at the mouth of the Moanda 
River. Several thousands of specimens collected by H. Lang. Landana, 
in the Portuguese colony of Cabinda, at the mouth of the Shiloango River 
(J. Bequaert Coll.). | | | 
_ As a rule, this species occurs together with Pachymelania fusca 
(Gmelin), from which it is, however, easily distinguished by the ex- 
panded lobe of the aperture and the distinct channel below the columella. 
The operculum is subcircular, with a regular, closely spiral figure starting 
from a central nucleus (Pl. XX, fig. 9). | 
A. d’Ailly (1896, Bihang Svenska Vet. Ak. Handl., Stockholm, 
XXIT, Afd. 4, No. 2, p. 121) lists among the synonyms of P. fuscatus also 
Murex cingulatus Gmelin (1791, in Linneus, ‘Syst. Nat.,’ 13th Ed., p. 
3961). From Gmelin’s reference to Pl. civ, fig. 1492 of Martini’s 
‘Conch. Cab.,’ IV, it appears that cingulatus is not an African snail, 
but the Oriental species usually called “fluviatilis.” Gmelin’s Murex 
fluviatilis, however, is a synonym of Potamides fuscatus, so that the 
Oriental species should be called Potamides cingulatus (Gmelin); it 
appears to belong in the subgenus T’ympanotonos. 
Dautzenberg and others have recognized the specific identity of the 
Linnean species fuscatus, in which there is a posterior series of stout 
spines on each whorl, and radula, which has a nearly uniform tubercula- 
tion. Intergradation is fully exhibited in the extensive material of the 
Congo Expedition. | 3 
In some lots, such as that from the mangrove swamps at the mouth 
of the Moanda River, both of the forms are found in about equal number, 
together with a smaller number of ‘transitional examples. A series from 
this place is illustrated in Plate XX, figures 1-6. Similar conditions are 
found in a lot from Malela, while another lot from the same locality, but 
from a different habitat, contains only the form radula and all the speci- 
mens are of small size. At Kunga and Banana, on the other hand, pure 
colonies of the form fuscatus were found. 
An extensive and very remarkable series of large-sized specimens, 
all of the radula form, was obtained at Bulabemba Island, near the 
lighthouse (Pl. XX, figs. 7, 8). They commonly reach 65 to 70 mm. in 
length, in some cases even up to 80 mm. Two specimens measured had a 
length of 78 and 61 mm. by a diameter of 27 and 26.5 mm. respectively. 
The photographs on Plate LX XVII give an idea of the tremendous 
‘numbers of these snails in the mangrove swamps of the Congo estuary. 
