1901.] OF THE GREAT AFKICAN LAKES. 465 



Not very far removed from this group, auci standing some\Ahat 

 in the manner of a stepping-stone between it and Tijpliobia^ ue 

 have the genera Tanganylcia and Giraudm, both molJuscs which 

 have a oirithoid organization, and the only members of the 

 lialolimuic group which could be considered under any circum- 

 stances as nearly related to Melania ; that is, if the oi-ganization of 

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

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

 closely related to Melania amarula as Geritldum ^ vulgatwn is, for 

 they possess the remarkable brood-pouches whicli 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 halolimnic series which we may call the Taur/ani/icia grouj). 



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

 of which I have described fully in the case of Tupliohia Jwarii'-^ and 

 Bathanalia howesi. Their organi^.ation is unquestionably similar 

 to that of the Aporrhaidse, the Strouibidae, and the Xeuophoridse, 

 all typical and I'airh^ old marine molluscan types, whii-h are 

 closely connected together, although Xeiwjy^iora has probably, 

 from conchulogical considerations, not hitherto been regarded as 

 nearly approaching either Aporrliais or Siromhus. Like the 

 marine types, the Typhobias are characterized by their raduloe, 

 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 obsei'ved that they lack the 

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

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

 such as StromJms, Pteroceras, AporrJiais, and Xenophora. 



During the second Tanganyika expedition, I obtained for the 

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

 Limnotrochus tJwmsoni and Kijtra Jcirki (PI. XXVI. fig. 2) ; and it 

 is extremely interesting to find that these two utterly enigmatical 

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

 genus Typliohia, together with the genera Bathanalia, Limnotrochus, 

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

 which I shall speak of as the Stromhoid section of the halolimnic 

 molluscs. 



The remaining halolimnic genera are Spekia and Nasso^^sis. 

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

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

 simplified Lamdlana without the specialization of the shell and 

 mantle peculiar to that form. Spjehia, then, constitutes in itself a 

 Naticoid section of the halolimnic molluscs. We have then lastly 

 Nassopsis; and this form, as I have shown, bears no proximate 

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

 Architeenioglossate t^^pes, such as Vivipava, which inhabit fresh- 



^ Proe. Bioy. Soe. vol. Ixix. p. 45L 



2 Moore, J. E. S. : Qii. Joiu-n. Micr. Sci. toI. xli. p. 181 



