1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 127 
Planorbis (Gyraulus) Avakubiensis, new species 
Text Figure 9 
Avakubi, in a woodland pool (J. Bequaert Coll.). 
The shell is about equally concave on both sides, thin, olivaceous, minutely, 
closely but not strongly striate, rather glossy. The whorls increase very rapidly. 
On the right side the concavity is rather funnel- or vortex-shaped, the whorls broadly 
rounded. The periphery is rounded, but a very little nearer the left side. The left 
side has a saucer-shaped concavity and rounded whorls. The aperture is large, its 
width more than one-third the total diameter of the shell, extremely oblique, cordate 
infront view. The peristome is thin; left margin curving strongly forward. 
Diameter, 2.7 mm.; altitude, 1mm.; barely 3 whorls. 
The rapid increase of the whorls to the large, strongly oblique aper- 
ture and the nearly symmetrical curvature of the periphery characterize 
this species. It appears related to P. lamyi Germain, of Lake 
Tanganyika, but that is a larger shell, said to be quite solid, and “avec 
une angulosité inférieure assez marquée.”’ 
Onc 
Fig. 9. Fig. 10. 
Fig. 9. Planorbis avakubiensis Pilsbry and Bequaert. Type. Avakubi. 
Fig. 10. Planorbis costulatus Krauss. Elisabethville. 
Planorbis (Gyraulus) costulatus Krauss 
Text Figure 10 
Planorbis costulatus Krauss, 1848, ‘Siidafrik. Mollusk.,’ p. 83, Pl. v, fig. 8 (type 
locality: Umgeni River, Natal). JIcKE Lt, 1874, Nova Acta Ac. Nat. Cur. Dresden, 
XXXVII, 1, p. 219, Pl. vu, figs. 22a-c (Abyssinia). ; 
Planorbis stelzneri E. v. Marrens, 1869, Malakoz. Blatter, XVI, p. 213 (Ailet, 
Eritrea) (not of Dohrn). 
Caillaudia angulata Bourauienat, 1883, Ann. Sc. Nat. Zool., (6) XV, p. 129 
(based upon Jickeli’s Pl. v1, figs. 22a-c of 1874). , 
- Kisanga River, an affluent of the Kafubo, near Elisabethville 
(Michael Bequaert Coll.). 
The very wide last whorl as seen from the right side, and the sculp- 
ture of fine, curved, radially retractive costule, with faint traces of 
spiral strie, are characteristic. The specimen agrees well with those from 
Abyssinia compared and also with Krauss’ figures and description. 
