78 rnocEEDiNGS of the malacological society. 



several species of which no cognizance is taken, and the writings of 

 Pelseneer and Nicolas also are not referred to. 



In the second place I propose to offer a few ohservations on the 

 supposed resemblance between some of the ' halolimnic ' species and 

 the Jurassic fossils with which they have been compared, and in this 

 place I may call attention to the fact that M. Bourguignat had, long 

 before the invention of the compound ' halolimnic,' employed the term 

 ' thalassoid ' with reference to these Tanganyika shells. Thalassoid 

 is a very descriptive word, and it is a pity, if its use in connection 

 with these molluscs was known to Mr. Moore, that it was not adopted. 

 It has been used by Professor von Martens and others. 



It might be interesting, if not useful, to speculate upon the cause 

 of so many shells from this lake having this marine aspect. Can the 

 quality of the water, as hinted at by Bourguignat (ii, p. 78), have 

 anything to do with it, or, as with a few exceptions all appear to be 

 littoral or shallow- water forms, have these thick shells been developed 

 to withstand the rough usage of the sui-f during storms ? It does not 

 follow, even if these species are the descendants of ancient marine 

 types, that they should have thick shells. Many fresh-water species 

 have excessively strong shells, Unios for example, whereas others, 

 which may be found in the same rivers, are very thin. It is the same 

 with many marine genera. Some of the species are strong and thick, 

 whereas others are comparatively thin ; but it is generally recognised 

 that species found on the shores between tide-marks and in shallow 

 water have stronger shells than those found in greater depths beyond 

 the reach of the waves' action. If the fact of these Tanganyikan 

 shells being met with near the shore has anything to do with their 

 solidity, we might expect to find the same thing obtain in other large 

 lakes, such as Nyassa and the Victoria Nyanza, but, with the exception 

 of one or two species in the former, such is not the case. There must 

 therefore be some other reason for this thickened thalassoid character, 

 and it may be that they are, as supposed by Mr. Moore, the descendants 

 of some very ancient stock which have retained in an exceptional 

 degree their marine facies. The thalassoid genera comprise those 

 forms which are found only in Tanganyika and have more or less the 

 appearance of being of marine origin, and it is curious that, as far as 

 we know them, they are restricted to the Gastropoda. None of the 

 Pelecypods exhibit other than a fresh-water aspect. The Gastropod 

 genera are about twenty-three in number, exclusive of about ten others 

 which I consider synonyms. Only of nine of these do we know 

 anything of the soft parts, and that almost exclusively from the 

 investigations of Mr. Moore and Miss Digby. It will be interesting 

 to know the conclusions which may be arrived at when these same 

 forms have been investigated by others, as they certainly will be some 

 day. The genera which have been studied are Tiphobia, Bathanalia, 

 Chytra, Limnotrochus, Bytlioceras, Paramelania, Tanganyicia, Lavigeria 

 { = JYassopsis), and Spelcia, and the following still remain to be investi- 

 gated, namely, Bridouxia, Baizea, Syniolopsis, Ance-ya, Giraudia, 

 Jouhertia, Randabelia, Leroya (?), Edgaria, Hirthia, Stanleya, Rumella., 

 Lechaptoisia, and Burtonilla. Of these Randahelia, Jouhertia, and 



