1927] _ Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo 499 
lying in the numerous crevices and a few live ones, mostly closed with 
calcareous epiphragms, were found unattached, nearly a oe down. 
The following species were obtained there: 
Achatina pferfferr Pseudopeas thysvillense 
~ LInmicolaria subconica germaini Streptostele zambiensis 
Homorus langi if congoris 
“ osborns Thapsia zambiensis 
Pseudoglessula hesset 3 Succinea congoensis 
Pseudopeas plebevum 
It may be noticed sha the genera show a general affinity to the 
West African fauna and that East and South African types, such as 
Helicide and Tropdophora, are totally absent. 
The various outcrops of dolomite in the Lower Congo should be 
carefully investigated malacologically. In the caves excavated in lime- 
stones at Thysville, Mr. H. Lang and the junior author found quite. an 
interesting fauna of land mollusks: Subulina glyptocephala, S. gratacapi, 
S. thysvillensis, Pseudopeas thysvillense, Curvella thysvillensis, Ptycho- 
trema bequaerti thysvillense, Gulella langi, Thapsia stanleyvillensis, and 
Succinea congoensts. 
The Southern Savanna District possesses also several species of 
Achatina (A. bandeirana, A. wildemant, and A. zebriolata) and Limi- 
colaria (L. congolanica, L. paludosa, and L. wathenensis) which have not 
been found elsewhere in the Congo, but whether these inhabit the 
savanna or occur only in forest galleries is at present unknown. 
Bukama, where the junior author resided in 1911, is a locality of 
great interest to the biogeographer because it is situated close to the 
southeastern border of the West African Subregion. It lies at an altitude 
of about 730 m., in approximately 9° 20’ S. and 29° 55’ E., near the 
southern end of the great alluvial plain of the Lualaba, a region of 
periodically inundated marshes, papyrus swamps, and lakes which fill 
the bottom of a rift valley known as the “Graben of Lake Upemba”’ 
(see p. 543). At Bukama itself West African or Guinean elements still 
predominate in the flora, as evidenced by dense groves of oi! palms 
(Elxis guineensis Jacquin) along the raised banks of the Lualaba. The 
following list of land mollusks obtained in that locality, though very 
small, clearly indicates the West African affinities of thefauna as a whole: 
Achatina rugosa Gulella levigata 
‘: glaucina Trochozonites percostulata 
Burtoa milotica nilotica — , _ Mesafricarion hahotides 
Limicolaria martensiana var. eximia Rachis braunsi 
«c c6 quadricingulatus (at 
Kalengwe) 
Subulina leva 
Pseudoglessula bowint 
Opeas venustum 
