LAKE TANGANYIKA — AN OLD JURASSIC SEA. 313 



other fresh-water stock in the general facies it presents. These 

 old cretaceous beds present the facies of a true fresh-water 

 fauna, otherwise they could not be identified as such ; they 

 contain no crabs or prawns, there are no impressions of jelly- 

 fish in the soft grey mud of which they are generally composed ; 

 they contain no shore sponges, Lamellariidse, no Xenophoridae 

 or other marine Gasteropods, all of which are still living in 

 the slightly brackish water of Tanganyika at the present time. 

 In fact, the halolimnic fauna differs from that occurring in 

 these fresh- water cretaceous beds just in those features which 

 distinguish fresh-water from marine stocks in general, and there 

 is not the slightest doubt that had the halolimnic fauna occurred 

 fossilised it would have been regarded as unquestionably marine. 



The halolimnic fauna of Tanganyika, then, is not the remnant 

 of a cretaceous fresh- water stock, neither is it like any cretaceous 

 marine fauna which we know, nor is it represented in any of 

 the upper Mesozoic beds. It is only when we compare the shells 

 of the halolimnic molluscs with those in several of the lowest 

 secondary formations that any substantial similarity appears. 



In fig. 1 A, are represented two remarkably fine examples 

 of the marine Jurassic genus Purpurina ; the figure is copied 

 from a specimen P. bellona courteously placed at my disposal 

 for this purpose by Mr. Hudleston from his magnificent collec- 

 tion of Jurassic fossils. The genus has a somewhat curious 

 history in literature, which will be found fully dealt with in 

 Mr. Hudleston's^ monograph, 'The Jurassic Gasteropoda.' As 

 amended in this work for P. elaborata, the diagnosis of the 

 genus runs as follows : 



" Shell ovate couoidal, apex acute, whorls about five or six, 

 posterior area tabulate, sides moderately tumid. The orna- 

 ments consist of about eighteen longitudinal costse, which are 

 feebly developed on the tabular area, rise up into spinous nodes 

 on the keel, and are strong and regular on the flanks of the 

 whorls. The costee have a tendency to die out anteriorly on 

 the body-whorl ; the costse decussate with regular and closely 



^ * A Monograph of the British Jurassic Gasteropoda.' Palseont. Soc, 1887, 

 Part 1, No. 2, p. 86. 



