1901.] OF TUB GREAT AFRICAN LAKES. 465 



Not very far removed from this group, and standing somewhat 

 in the manner of a stepping-stone between it and Ih/phobia, we 

 have the genera Tanganyicia and Giraudia, both molluscs which 

 have a oirithoid organization, and the only members of the 

 haloliinnic group which could be considered under any circum- 

 stances as nearly related to Melania ; that is, if the organization of 

 Mt lania amarula, Lamarck's type of the genus, is considered typical 

 of that group. Tanganyicia and Giraudia are, however, not so 

 closely related to Melanin amarula as Gerithium l vulgatum is, for 

 they possess the remarkable brood-pouches which I have described, 

 features which are quite peculiar to themselves and to certain 

 widely divergent molluscan types. Their stomachic apparatus is 

 also characterized by the presence of crystalline style-sacs and 

 crystalline styles. These molluscs form, then, a second subgroup 

 of the haloliinnic series which we may call the Tanganyicia group. 



Next to them we come to the unique Typbobias, the anatomy 

 of which I have described fully in the case of Typhcibia hoarii 3 and 

 BaihanaUa howesi. Their organization is unquestionably similar 

 to that of the Aporrhaidse, the Strombida?, and the Xenophorida\ 

 all typical and fairly old marine molluscan types, which are 

 closely connected together, although Xenophora has probably, 

 from conchological considerations, not hitherto been regarded as 

 nearly approaching either Aporrhais or Strombus. Like the 

 marine types, the Typbobias are characterized by their radulae, 

 their nervous system, and the presence of crystalline styles 

 and style-sacs in their stomachic apparatus, as well as by their 

 reproductive organs. But it will be observed that they lack the 

 peculiar specialization of shell, foot, and operculum which cha- 

 racterise the modern representatives of this great group in the sea, 

 6uch as Strombus, Pteroceras, Aporrhais, and Xenophora. 



During the second Tanganyika expedition, I obtained for the 

 first time abundant material for the study of the two unique shells 

 Limnotrochus thomsoni and Kytra Tdrki (JP1. XXVI. tig. 2) ; and it 

 is extremely interesting to find that these two utterly enigmatical 

 Lake-forms are both close allies of the Typbobias themselves. The 

 genus Typhobia, together with the genera Bathanalia, Limnotrochus^ 

 and Kytra (PL XXVI. fig. 2), therefore make up a fourth group, 

 which L shall speak of as the SCromboid section of the balolimnic 

 molluscs. 



The remaining haloliinnic genera are SpeJeia and Nassopsis. 

 The first of these, SpeJeia, is, as I have shown, a Naticoid, which 

 appears From the minute details of its nervous system to be a 

 simplified LameUaria without the specialization of the shell and 

 mantle peculiar to thai form. SpeJeia, then, constitutes in itself a 

 Naticoid section of the balolimnic molluscs. \\'c have then lastly 

 Na$80psis\ and this form, as I have shown, hears no proximate 



relationship either to any forms living in the sen or to the 

 Axchitenioglossate types, such as Vivipara, which inhabit fresh- 



1 rro.\ Roy. Boc. rol. lxii. p. 151 . 



3 : Qu. Jen.- ' 3d. vol. xh. p. 181 



