308 J. E. S. MOORE. 



which, judged by their anatomical (as well as by their con- 

 chological) features, do not appear to be living elsewhere now, 

 but their shells approximate in a most remarkable degree to 

 those of the extinct marine Jurassic genus Purpur in a, whilst 

 at the same time they possess the nervous system of a Cyclo- 

 phorus. They thus appear not only to come of a marine 

 stock, but also to indicate the hitherto unknown road by which 

 the Cyclophoran nervous system has been evolved. These living 

 Tsenioglossa stand in much the same relation to their extinct 

 marine ancestors as the living Cyclostoma has been shown 

 (by the beautiful investigations of Lacaze-Duthiers and Bouvier) 

 to stand in relation to the common periwinkle of our shores. 



No Stromboid, Naticoid, or Xenophoran molluscs have 

 been found hitherto in any fresh water that is known ; and 

 when we remember that these truly marine Gasteropods are 

 associated in Tanganyika with other and widely different 

 marine forms, such as sponges, medusce, crabs, and prawns, it 

 is impossible to avoid the conclusion that these animals can be 

 anything but the dwarfed and stunted remnant of a fauna that 

 the sea has left behind. 



That the halolimuic animals still living in Tanganyika are the 

 remains of an extensive sea fauna that once existed there, is thus 

 the plain and unequivocal testimony afforded by the morphology 

 of the widely different types of which this fauna is composed. 



This being so, in the present paper I shall attempt to show, 

 from a variety of considerations relating to the similarity 

 between the halolimnic shells and certain fossils, and upon 

 more general grounds, to what old sea fauna the halolimnic 

 series once belonged. It will probably have been seen that 

 the above conclusion by no means exhausts the information 

 which can be gathered from the joint study of the distribution 

 and the morphology of the halolimnic group. We need only 

 refer to what is now generally known respecting the gross 

 physical features of the African continent, and especially of 

 the regions about the great lakes, to see that it is impossible 

 for most if not all the halolimnic forms either to have made 

 their way up to, or to have been left in, Tanganyika in recent 



