i6 



ORIGIN OF FRESH- WATER UNIVALVES 



CHAP. 



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 (^ Tele sc opium, Pyrazus, Pirenella, 

 Cerithidea, etc.), all of which inhabit swamps 



Fig. 11. — A, CoraineUa, 

 a marine genus, 



which lives between ^nd mudflats iust above high- water mark in all 

 which is probably warm countries, are derived from Cerithium 

 derived B, Clea, a (Fig. 12) ; Assimiuea, HydroUa, and perhaps 



genus occurring only ^ no rt- -ti • iii 



in fresh water. TruncateUa, irom Kissoa. It is a remarkable 



fact that in Geomelania (with its subgenera 



Chittya and Blandiella) we have a form of Truncatella 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. 

 CremnoconcJms, a remarkable shell occurring only on wet cliffs 

 in the ghats of southern India, is a modified Littorma. 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. lineata, 



1 Not to Xassa, as has been generally held. The shape of the operculum, 

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

 Cominella. 



