MOLLUSCS 173 



Egypt, iii which remains of Siluridce are abundant, it seems not improbable that 

 while barbels and their kindred are an Asiatic type, cat-fishes may have originated 

 in Africa. 



The balti {Tilapia nilotica), belonging to the family Cichlidce, is another well- 

 known Nile fish which grows to a large size. 



A small fish from the rivers of West Africa, measuring about three inches in 

 length, and provided with elongated pectoral fins, is in the habit of taking aerial 

 flights after the fashion of the true flying-fish and flying-gurnards. This fish 

 (Pantodon bucholzi) is the only fresh-water species known to possess powers of this 

 nature. Although allied to the Osteoglossidce, it is regarded as the representative 

 of a special family, the Pantodontidce. 



The remarkable absence of eels from the rivers of western Africa has been 

 already referred to in the chapter on North Africa, where mention is also made of 

 the species found in the rivers of other parts of that continent. 



Ethiopian Africa as a whole is characterised by the poverty of 



Molluscs. r . . . 



its fauna of land-snails and slugs in comparison with other tropical 

 regions. Nevertheless, it is, with the exception of a few Malagasy forms, the 

 exclusive home of the agate-snails, as typified by the genus Achatina, but including 

 also a number of other generic types which it is unnecessary to particularise. 

 Another feature is the scarcity or poor development of the typical genus Helix 

 and its relatives, collectively constituting the family Helicidce, the same remark 

 applying in the case of the Naninidce. It is not, indeed, that the Helix group is 

 entirely absent, for it attains a considerable development in the southern part of 

 the continent, but the African species are nearly all small and dull-coloured. In 

 this respect Ethiopia presents a striking contrast to Madagascar, where there are 

 many large and handsome representatives of the group ; and it would accordingly 

 seem that while the Achatina, are an original African group, the Helicidce are 

 immigrants into Ethiopia. On the other hand, in possessing a certain number of 

 representatives of the genus Gyclostoma, as now restricted, which is peculiar to 

 this region and Madagascar, Ethiopian Africa exhibits affinity with that island. 

 The genus belongs to a family characterised by the mouth of the shell being closed 

 by a horny operculum. These snails are absent from the west coast. 



The largest species of Achatina, and, in fact, the largest land-snail in the 

 world, is the orange and brown A. tnarginata of the west coast, the shell of which 

 attains a length of 8 inches. In 1908, living specimens of these giant snails were 

 exhibited at the Natural History Branch of the British Museum. Four specimens 

 were received at the museum, all of which on arrival had their mouths sealed by 

 a stout " epiphragm." The two exhibited were induced to become active by 

 immersion in warm water, and appeared to be in full health when placed in the 

 glass jar in which they were shown. Very interesting was it to watch the curious 

 ripple-like movement in the foot as they crawled up and down the sides of their 

 cell. It was also noticeable that a very large proportion of the soft-parts was 

 protruded from the shell. The zebra-snail (A. zebra) of southern and western 

 Africa is remarkable for possessing a type of colouring and markings recalling 

 those of GreVy's zebra. Another well-known species is A. fulica, a native of East 

 Africa, which was introduced many years ago into Mauritius, where it is now 



