LAKE TANGANYIKA — AN OLD JURASSIC SEA. 305 



It is perhaps needless for me here to reiterate the great im- 

 portance of arriving at a final decision as to the real nature of 

 the haloliranic forms, for it will be obvious that if they have 

 nothing to do with the normal fresh-water series, and are to 

 be regarded as the remnant of an ancient sea, our views 

 respecting the past history of the African interior must be 

 greatly changed. 



Having obtained the animals, it appeared, therefore, in tiie 

 first place to be incumbent on me to ascertain, by a careful 

 study of their anatomy, whether this superficially marine 

 appearance was real, and indicative of their common origin 

 from the sea, or whether it was merely, so to speak, skin deep, 

 and to be regarded as wholly the result of modification, or to 

 the persistence of characters which belonged to some old fresh- 

 water stock. So far as zoologists are concerned, the evidence 

 which I have now accumulated on this point will be found to 

 be conclusive ; but since, with the exception of my paper on 

 the Typhobias (loc. cit.), the detailed accounts of the anatomy 

 of the Halolimnic Gasteropods have not yet been published, I 

 will briefly recapitulate the facts. It has been found ^ — 



^ In order that the significance of tiie new classificatoiy outline, given in the 

 text immediately below, may be fully appreciated, it should be clearly under- 

 stood that before my return from Tanganyika no account of the anatomy of any 

 of the halolimnic molluscs was in existence, — indeed, so far as I can ascertain, 

 with the exception of the brief description by Smith of a few badly preserved 

 Nasopses brought home by Captain Hore, the animals contained in any of 

 these shells had never been seen before. Consequently their conchological 

 classification was, as, indeed, Smith frankly implied in 1881, entirely pro- 

 visional. 



All these Halolimnic Gasteropods appear to be rigidly restricted to the 

 confines of Lake Tanganyika, and the only molluscs from this lake, halolimnic 

 or otherwise, of which we have had any anatomical description, is the common 

 Reiodon, the morphological characters of which were described by Professor 

 Pelseneer from specimens which were brought back by the oflicials of the 

 Congo Free State (' Bull, de Musee Royale d'Hist. Nat. de Belg,.' Brussels, 

 1886, iv, p. 103). 



The marine appearance of the genus Nassopsis was pointed out by 

 S. P. Woodward in 1857, but with curious inconsistency he regarded this 

 form as a Melania belonging to the sub-genus Melanella. It is to Smith that 

 we owe the first definite assertion of the possibility that these Gasteropods 



