i6 



ORIGIN OF FRESH-WATER UNIVALVES 



reason why the ancestry of <ill I'uhuonata, whether land or fresh- 

 water, is so (lifficnlt to tract'. 



(h) Gasteropoda. — (I) Oprrculatc. Caniducnnd C/ea are 

 closely allied, with but little nioditication, to 

 the marine Cominellu'^ (i^g- H)^ '^^ i^ <''l^'* 

 JSfassodonta to JVassa. They occur (in fresh 

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

 and Borneo, associated with essentially fresh- 

 water species. Potamides, witli its various 

 subgenera {T el esc opium , l^yrazvs, Pirenella, 

 ^a marine ^eii us, which Gerithidea,e.tc.),'A\\ ofwhich inhabit swamps and 

 lives between tide mudflats just above high-water mark in all warm 



marks, and from ' i • i\- n -±1 ■ /i.-'- i o\ 



which is proLahiy Countries, are derived irom Certt/num (1' ig. 12): 

 derived B, Clea, ^i Assim i Hca, Jli/drohia, ixiid -^erha^s Tru7icateU<i , 

 from Itissoa. It is a remarkable fact that in 



genus occiuTing only 

 iu fresh water. 



Geomelania (with its subgenera Chittya and 

 JJlandieUa) we hnxo a form of Truncatclla which lias entirely 



Fig. 12. — A, Cerithium coluvma Sowh. (marine). B, Potamides microptera 'K\n\. 

 (brackish water). C, lo tijjiuosa Lea, one of the Pleuroeeridae (fresh water). 



deserted the neighbourhood of the sea, and lives in wood}' 

 mountainous localities in certain of the West Indies. Cremow- 

 conchvs, a remarkable shell occurring only on wet cliffs in the 

 ghats of southern India, is a modilied Littorina. 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 {JSF. lineata, Cheni.) ascends 



^ Not to JVassa, as lias been generally held. The shape of the operculum, and 

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



