THE MOLLUSCS OP THE GREAT AFRICAN LAKES. 201 



Islands, the small but quite apparent siphonal extension of 

 the mantle represented in fig. 4. This Melania had in every 

 other respect the true characters of the group, but from the 

 existence of the siphon it would, according to the old arrange- 

 ment, have to be removed from the Melaniidse and associated 

 with those families of the Tsenioglossa to which it most cer- 

 tainly does not belong. The distinct but small anterior pro- 

 longation of the mantle in Ty phobia (fig. 3, 1), does not 

 therefore appear to be of primary morphological importance, 

 but its existence is undoubtedly another indication of the 

 general similarity of the Typhobias to the forms which I have 

 named. 



The reproductive apparatus in the Typhobias has been pro- 

 bably much modified through changed conditions, and the 

 peculiar position of the penis is possibly more the result of 

 extreme specialisation than the retention of any primitive con- 

 dition. 



From all this it will be seen that the Typhobias can hardly 

 be said to be archaic forms, but they do, as in the character of 

 the nerves and the otocysts, possess some undoubtedly archaic 

 characters. They are far less specialised in the characters of 

 the foot and mantle than Strombus and Pterocera, to 

 which in other respects they appear to be closely allied. 

 They certainly possess none of those characters which would 

 suggest that they can by any possibility be regarded as the 

 persistent representatives of an old fresh-water stock. They 

 do, however, simulate and retain the characters of the nerves 

 of the Solarium and the Scalarids, and they probably indi- 

 cate the road by which the more modern marine genera of the 

 Strombidae and their associates have been evolved. But to my 

 mind the most remarkable features which they present are 

 those which I have pointed out as indicating an approxima- 

 tion to several forms which have been generally regarded as 

 recent productions; that is, they distinctly bridge the gap be- 

 tween several twigs which are well up in the phylogenetic 

 tree. 



Lastly it will have been seen that in many ways the 



VOL. 41, PART 1. NEW SERIES. O 



