THE MOLLUSCS OF THE GREAT AFRICAN LAKKS. 179 



number of the Melaniidse possess^ for there are, as we have seen, 

 two distinct types of so-called fresh-water Melanias; one charac- 

 terised by the possession of a Cerithio-planaxoid type of radula, 

 the other by the dentition of the modern Littorinoid group. 



Now if Bouvier's view that the Planaxoidse are related to 

 the Littorinidse be correct, the modern Littorinas must have 

 become modified in their teeth, for the Planaxoid radula is the 

 older of the two (see p. 174). It follows consequently that 

 those Melanias which possess a Cerithio-planaxoid radula are 

 relatively the older fresh-water types ; but they may be con- 

 sidered as having arisen directly either from the marine Ceriths 

 themselves, or from a Littorino-planaxoid of the older type. 

 In whatever way we view tiiis matter, it is at any rate obvious 

 that the Melaniidse are no real family group, but can certainly 

 be split into two broad divisions, one of which originated as a 

 fresh-water stock from the more modern Littorinas, and may 

 for present purposes be called the Littoriuo-melanoid 

 group. The other has a Planaxoid radula, and the exact 

 origin of its constituent forms is much more difficult to deter- 

 mine, for in the characters of their radulse these species are 

 equally similar to both the Cerithidse and the Planaxidae, nor 

 are there any other features at present known by which, 

 in the majority of cases, it would be possible to determine 

 satisfactorily from which oceanic stocks such Melanoids origi- 

 nally sprang. Unquestionably the shell and the internal 

 anatomy of many Melanias correctly indicate a Cerithioid 

 ancestry, as Bouvier supposed ; but there are others, such as 

 M. episcopalis, which might equally well be supposed to 

 have originated from the older types of Littorinas which 

 possessed a Planaxoid radula, or even from the Planaxidse 

 themselves. Until further investigation has been prosecuted 

 it seems, therefore, best to group all those Melanias with a 

 Planaxoid radula, whatever their origin, into one series, which 

 we may describe as tlie Melanio-planaxoid group. 



In T. rufofilosa we have a form which in many ways would 

 typify the marine ancestry of many Melanio-planaxoids, and 

 the exact relation of this form to the living oceanic Planaxidae 



