1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 503 
luxuriant and extremely dense growth of elephant grass (Pennisetum 
purpureum Schumacher and Thonning) reaching 4 m. to 6 m. in height. 
The grass stalks are well sheltered at their base and the humid, hot 
atmosphere produces there rather favorable conditions for snails and 
slugs. One can usually find in these grass thickets Vaginulide, Heli- 
xarionine, Ledoulxia, and Thapsia; sometimes also some of the Streptaxi- 
de. Halolumnohelix intonsa Pilsbry, a true helicid, was found in one of 
these growths of Pennisetum, at Boga, in about 1,000 m. The beautiful 
Burtoa nilotica (Pl. LV) is by no means rare in this district. Native 
plantations of bananas are often located on such fertile soil and, as usual, 
they contain a number of widely distributed snails and slugs. But by 
far the richest habitat, malacologically speaking, are the forest galleries 
which follow the water courses across the savanna. In some parts these 
galleries consist only of very few bushes and trees, but elsewhere they are 
quite extensive and present many of the environmental features of true 
rain forest. Near Beni a comparatively narrow forest gallery, shown on 
Plate LXIX, fig. 1, yielded numerous specimens of Homorus amputatus, 
Ptychotrema runsoranum, Gulella linguifera, and G. medzafricana. The 
narrow band of forest along the Rutshuru River, a short distance south of 
Rutshuru, possesses an extremely rich fauna, as may be gathered from the 
subjoined list. The exceptional abundance of land mollusks in that 
locality is evidently due to the high proportion of lime present in the soil, 
a fact to which attention has been called above (see p. 467). 
Halolimnohelix rutshuruensts Gulella rutshuruensis 
Homorus clarus yes @sbornt 
Pseudoglessula umbilicata Thapsia rutshuruensis 
ey subfuscidula i cinnamomeozonata 
ms cruda e consobrina 
Pseudopeas plebecum t¢ rufescens 
Curvella lang Helixarion insularis 
Gonazxis cavallit tturvensis | Atoxon flavum rutshuruense 
Varicostele rutshuruensis Dendrolimax osborna 
Ptychotrema geminatum Trichotoxon maculatum perforatum 
. nyangweense | Pachnodus rutshuruensts 
Gulella disseminata cymatonotus Lezvicaulis schnitzlera 
A comparison of this fauna with that of the Lower Guinea Forest 
District, as exemplified by the list given above for the rain foresi at Penge, 
well brings to light its West African affinities. The only striking differ- 
ence is the presence of a true helicid. The occurrence of Helicide in the 
forest galleries of the Uganda-Unyoro Savanna District is by no means - 
exceptional. Thus Halolimnohelix sericata was also found crawling over 
