232 PEOCEEDINGS OF THE MAIACOIOGICAL SOCIETY. 



scalariform pedal cords} Moreover, we find in JVaftsopsis that the 

 buccal mass and the salivary glands are similar to those of Littorina 

 or Vivipara, while the stomach is extremely peculiar. This organ 

 is in the first place divided into two chambers, into one of which 

 the oesophagus, intestine, and liver open, while the other has thick 

 walls, is lined internally with a singularly iridescent membrane, and 

 contains an almost globular crystalline style. Furthermore, there 

 appears on the postero-ventral wall of the stomach a spirally arranged 

 valvular structure, which I am disposed to regard as homologous 

 with the spiral caecum so frequently present in the Rhipidogiossa. 



Similar primitive features are exhibited in the genital apparatus, 

 for we find that there are no accessory genital organs, and that the 

 animal is viviparous. 



Thus, so far from Nassopsis conforming to the true Melanian tj-pe, 

 it is seen upon anatomical examination to be a highly peculiar and 

 primitive form ; a form which I believe to be on the whole more 

 primitive and more typically representative of the Archi-tsenioglossa 

 than the genus Vivipara itself. 



The existence in this tsenioglossate of what I believe to be the 

 rhipidoglossate spiral cascum is something quite new, and must be 

 regarded as of great interest in connecting the Archi-taenioglossa 

 with the Rhipidogiossa. 



I have already pointed out^ that the shell of Nassopsis, like so' 

 many of the halolimnic Tanganyikan types, is indistinguishable from 

 the Jurassic Furpurina Bellona, D'Orb., so that, viewed from what- 

 ever side we will, Nassopsis appears persistently to be a form which 

 has no connection with the Melaniidse, and belongs to an extremely 

 old type. I have, therefore, felt justified, at any rate for the present, 

 in including it among the otherwise extinct Purpurinidse. 



In Bythoceras, another mollusc from the deep water of Lake 

 Tanganyika, we have a form which, judged by the shell and operculum, 

 appears to be closely related to the genus Paramelania. Paramelania 

 has always been considered by conchologists to be closely related to 

 Nassopsis ; but if it bears any relation to Bythoceras — and the reverse 

 is almost inconceivable — then it can have no affinities whatever with 

 Nassopsis, because Bythoceras is found to have the general anatomy, 

 the nerves, the alimentary canal, and pallial complex of the true 

 Ceritho-Melanian group, while in certain features of its radula Bythoceras 

 is almost indistinguishable from the marine genus Tympanotomus. 



Thus it is apparent that the genera Typhobia, Bythoceras, Nassopsis, 

 and Melanopsis not only differ as widely from each other as members 

 of so many distinct families, but that, with the exception of 

 Bythoceras, they bear not the slightest resemblance to the typical 

 Melanian group, and must unquestionably be expunged from it. 



^ It will have been noted that the European Vivipara is dyaloneurous in Bouvier's 

 sense ; but I find that in the Tanganyikan genus Neothauma (the generic 

 distinction of which from Vivipara Dr. Pelseneer regards as superfluous), the 

 right side of the nervous system is zygoneurous, as in Nassopsis. 



2 Quart. Journ. Micros. Sci., vol. xli (1898), p. 303. 



