1927] Pilsbry-Bequaert, The Aquatic Mollusks of the Belgian Congo O85 
fresh-water animals living at present. Many of these belong to endemic 
genera and in a few cases to peculiar families or subfamilies. The 
endemic mollusks are especially striking, often showing all the outward 
appearances of oceanic types and consequently Bourguignat proposed 
calling them “thalassoid”’ or marine-like,! an appropriate term which, 
moreover, does not commit us to any view as to the possible origin of 
these animals. : | 
There are no endemic forms among the few mammals, crocodiles, 
turtles, and amphibians known from the lake, but of the five water-snakes 
recorded two are peculiar; one of these belongs to an endemic genus, 
while the other possesses a relative in Nyasaland. The fishes are better 
represented than any other group, no less than 146 species, belonging to 
55 genera being known at present, and of these 121 or 88%, belonging to 
26 genera, are endemic. ‘These fishes pertain to eleven families which, 
however, are all typical fresh-water groups or at least contain elsewhere 
fresh-water forms. The Cichlide are extremely well represented by 89 
species, 84 of which are endemic. The Polyzoa, though only five in 
number, are of especial interest: at least two of them are endemic, while 
another has only been found in India. Three belong to the Phylactole- 
mata, a group generally found in fresh-water; the two others are Gym- 
nolemata, the members of which are, with few exceptions, marine and it 
is of some significance that both belong to genera (Victorella and 
Arachnoidia) that are represented by living, true marine species in the 
East Indies. Among the Crustacea, the Macrura are represented by 
twelve species of prawns (of four genera), all of which are endemic, 
although they belong undoubtedly to fresh-water groups; the Brachyura 
contain five species of crabs (of two genera), four of which are endemic, 
. but in this case too they are allied to fresh-water types. ‘The groups of 
smaller Crustacea (Eucopepoda, Branchiura, and Ostracoda) are repre- 
sented by many species (62, of which 49 are endemic), but none of them 
show any affinity to marine forms. The six species of Hydrachnida 
(three endemic); three species of Oligochzta (two endemic), five or six 
species of Hirudinea (leeches), twenty-nine species of Rotifera (one 
endemic), one endemic species of Turbellaria, six species of Porifera (four 
endemic), and seven species of Protozoa (one endemic) are all essentially 
fresh-water types. 
1J. R. Bourguignat. 1885. ‘Notice prodromique sur les mollusques terrestres et fluviatiles re- 
cueillis par M. Victor Giraud dans la région méridionale du lac Tanganika.’ (Paris), p. 9. 
We shall use the term ‘‘thalassoid’”’ in preference to that of “‘halolimnic,” which was proposed 
later by J. E. S. Moore for the same mollusks. The latter designation implies a direct marine origin of 
these mollusks, independent from that of the usual fresh-water fauna, a theory which will be discussed 
in the sequel. 
