1927]. ~—- Palsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 215 
tional correction, and as the first binomial author to utilize Adanson’s 
pre-Linnean term, Scopoli’s right to modify the spelling and the scope 
of the genus cannot be questioned. Since Succinea and Lymnea are 
the oldest and most widely used of the three above mentioned names, we 
propose to take Helix tentaculata Linneus as the type of Scopoli’s 
Bulimus which consequently replaces Bithynia Leach. 
This genus is abundantly developed in the Old World, especially in 
its warmer portions. One European form has been introduced into North 
America. But few species are thus far known from the Ethiopian 
Region and none from Madagascar. 
Subgenus Gassia Tryon | 
Gabbia Tryon, 1865, American Journ. Conch,, I, p. 220. Monotype: Gabbia. 
australis Tryon. 
Digyrcidum Locarp, 1882, ‘Prodr. Malacol. Frang., Moll. Terr. Eaux Douces,’ 
p. 224. Monotype; Bythinia bourguignati Paladilhe. 
Digyreidum Locarp, 1893, ‘Coq. Eaux Douces et Saum. France,’ p. 75. Mono- 
type: Bythinia bourguignati Paladilhe. 
Diggreidum Pmret, 1887, ‘Catal. Conch. Samml.,’ I, p. 428. Misspelling of 
Digyreidum. | 
The form is like Bulimus; the outer lip may be a trifle thickened 
outwardly at the edge, but not expanded. The operculumlodges at the 
edge of the aperture, and has a larger spiral portion than in Bulimus 
proper. | 
Gabbia is here understood to include Digyretdum (=Digyrcidum), 
in which the spiral part of the operculum is typically larger than in B. 
tentaculatus, occupying the greater part of the whole. In African species 
the size varies; B. senaariensis sometimes has the spiral part as small 
as in the type of Gabbia, while B. kisalensis has it as large as in that 
of Digyrerdum. : 
It appears that the operculum of Bulimus, in the embryonic stage, 
is spiral. The spiral mode of increase may continue in the neanic stage 
for a time, varying in different species, after which the operculum ceases 
to revolve with growth, the increment becoming concentric, nearly equal 
on all sides. In B. tentaculatus the spiral portion is very small, yet 
distinctly visible in unworn opercula. In the species upon which Gabbia 
and Digyreidum were founded, the spiral portion is larger and therefore 
more readily seen. The concentric stage is a highly specialized structure, 
superposed upon the earlier, more primitive spiral type, common to 
other genera of the family. 
