ORIGIN OF FRESH-WATER UNIVALVES 



reason why the ancestry of all Pulmonata, whether land or fresh- 

 water, is so difficult to trace. 



(5) Gasteropoda. — (1) Operculate. Canidia and Clea are 

 closely allied, with but little modification, to 

 the marine Cominella'^ (Fig. 11), as is also 

 Nassodonta to Nassa. They occur (in fresh 

 water) in the rivers of India, Indo-China, Java, 

 and Borneo, associated with essentially fresh- 

 water species. Potamides^ with its various 

 subgenera (Telescopium^ Pyrazus^ Pirenella, 

 a marine genus, Cerithidea^ etc.), all 01 which inhabit swamps 

 which lives between ^nd mudflats lUst abovc high- water mark in all 



tide marks, and from ^ . i • i (• /y -j^i • 



which is probably warm countries, are derivea irom (Jeritfiium 

 derived B, Clea, o. (Fig. 12) ; Assimifiea, Hydrohia, and perhaps 



It is a remarkable 



genus occurring only . n £ t>- 



in fresh water. TruneateUa^ irom Kissoa. 



fact that in Gieomelania (with its subgenera 

 Chittya and Blandielld) we have a form of Tnmeatella which 



Fig. 12. — A, Cerithium columna Sowb. (marine). B, Potamides microptera Kien. 

 (brackish water). C, lo spinosa Lea, one of the Pleuroceridae (fresh water). 



has entirely deserted the neighbourhood of the sea, and lives in 

 woody mountainous localities in certain of the West Indies. 

 Oremnoconchus^ a remarkable shell occurring only on wet cliffs 

 in the ghats of southern India, is a modified Llttorhia. Neritina 

 and Nerita form a very interesting case in illustration of the 

 whole process. Nerita is a purely marine genus, occurring on 

 rocks in the littoral zone ; one species, however, (iV. Uneata, 



1 Not to N'assa, as has been generally held. The shape of the operculum, 

 and particularly the teeth of the radula, show a much closer connexion with 

 Cominella. 



