THE MOLLUSCS OF THE GREAT AFRICAN LAKES. 193 



member of the halolimnic fauna of that lake is of considerable 

 interest. Firstly, because, although conchologically so similar 

 to the genera Nassopsis and Paramelania, it has, as we 

 shall immediately see, no morphological relation to the former 

 of these types. By thoceras is interesting, secondly, because 

 it presents us with more numerous points of correspondence 

 with forms such as the genus Tympanotamus, which exists 

 elsewhere, and the anatomy of which is known, than is the 

 case with the majority of the halolimnic forms. By thoceras 

 is at present represented in Tanganyika by a single species, 

 B. iridescens, which I dredged living at great depths^ in the 

 southern portion of the lake. When young the shell does not 

 possess the characteristic spines represented in PI. 21, fig. 3 

 (compare Pi. 3, fig. 4). Nor has it the peculiar pearly thicken- 

 ing of the mouth invariably present in the older forms. In the 

 young condition (PI. 21, fig. 4) the shell is extremely similar to 

 that of Paramelania, and I am inclined to think that the 

 figure of Paramelania crassilabrio given by Professor E. 

 von Martens in his work ' Beschalte Weichthiere, Deutsch. 

 Ost-Afrikas ' (PI. vi, fig. 38), is, in reality, that of a young 

 Bythoceras iridescens. 



The outward appearance of the animal is extremely similar 

 to that of Cerithium vulgatum, with the exception that 

 there is less pigmentation of the foot, which is nearly white 

 in the Tanganyika species. 



The snout is short, wrinkled, richly covered with black 

 pigment, and non-protrusible. The tentacles are short, and 

 the eyes are situated on the posterior bases of these organs, 

 and not separated from them on subsidiary papillae as in 

 Nassopsis. The buccal mass is small, and the radular sac 

 short, being reduced, as in the case of Ty phobia and Tan- 

 ganyikia, to a small swelling on the floor of the oesophageal 

 tube (PI. 21, figs. 7— 11, r.s.).2 



The radular dentition is extremely interesting^ a single row 



1 From 300 to 1000 feet. 



^ Compare figures of Tanganyikia r uf of 11 osa, preceding article (loc. 

 cit.). 



