320 J. E. S. MOORE. 



species. We are sure, therefore, that the haloliranic group 

 represents an old sea stock that became detached from the 

 general oceanic fauna of which it was a part, far back in time. 

 Like the Oolite molluscs, those of this halolimnic fauna have 

 a striking type of shell, and when, after reviewing the facies 

 presented by the marine fauna of the successive geological 

 periods, we find such types represented abundantly nowhere 

 except in the Jurassic seas, and that these seas present forms 

 corresponding to them all, the comparison appears to be some- 

 thing more than a mere coincidence. It rather appears as the 

 fulfilment of an ex|>ectation raised simultaneously by the three 

 chief lines of search relating to their distribution, morphology, 

 and affinity with existing types. 



I offer this comparison, therefore, as the probable explana- 

 tion of the singularly interesting problem presented by the 

 mixed fauna which Tanganyika now contains, and I have all 

 the more confidence in so doing since much study of the 

 question, in the light of every suggestion which I could either 

 invent or borrow, has convinced me that no other even mo- 

 mentarily tenable explanation is likely to be found. 



DESCRIPTION OF PLATE 23, 



Illustrating Mr. J. E. S. Moore's paper " On the Hypothesis 

 that Lake Tanganyika represents an Old Jurassic Sea." 



Fig. 1. — Front and back view of the shell of Paramelauia Damoni 

 (Smith), Tanganyika. 



Fig. 1a. — Front and back view of the shell of Purpuriua bellona (Hudl.), 

 the corresponding Jurassic form. 



Fig. 2. — Front and back view of the shell of Nassopsis nassa (Smith), 

 Tanganyika. 



Fig. 2a. — Front and back view of the shell of Pur p urina inf lata (Hudl.), 

 the corresponding Jurassic form. 



