I ORIGIN OF FRESH-WATER BIVALVES 1 5 



and lives in the Ganges, the Jumna, and the Tenasserim at 

 a distance of 1600 miles from the sea (Fig. 9). Pliolas ^nvicola 

 is found imbedded in floating wood on the R. Pantai many 

 miles from its mouth. Cyrena^ Corhicula, and probably Sphae- 

 rium and Pisidium are derived, in different degrees of removal, 

 from the exclusively marine Veneridae ; Potamomya (rivers of 

 S. America), and Himella (R. Amazon) are forms of Corhula. 

 The Caspian genera derived from Oardium (^Adacna, Dldacna, 

 Monodacna)^ have already been referred to. Nausitora is a 

 form of Teredo^ which lives in fresh water in Bengal. Rangia^ 

 Fischeriay and G-alatea probably share the derivation of the 

 Cyrenidae, while in Ipliigenia we have one of the Donacidae 

 which has not yet mounted rivers, but is confined to a strictly 

 estuarine life. The familiar Scrohicularia piperata of our own 

 estuaries is a Tellina, which lives by preference in brackish 

 water. 



The great family of the Unionidae is regarded by Neumayr^ 

 as derived from Trigonia^ the points of 

 similarity being the development of a 

 nacreous shell, the presence of a strong 

 epidermis, and the arrangement of the 

 muscular scars. It is remarkable, too, 

 that on many Uniones of Pliocene times 

 there is found shell ornamentation of such 

 a type as occurs elsewhere among the 

 Pelecypoda only on Trigonia. 



The genera of fresh-water Pelecypoda 

 are comparatively few in number, and ^^^- ^'^• — Ti^iyomapec- 



,, . . . . p -^ TIT -11 tiaata Lam., Sydney, 



their origin is tar more clearly discernible n.s.w. 

 than that of any other group. This is 



perhaps due to the fact that the essential changes of structure 

 required to convert a marine into a fresh-water bivalve are but 

 slight. Both animals "breathe water," and both obtain their 

 nutriment from matter contained in water. Similar remarks 

 apply to fresh-water operculate Gasteropoda. But the passage 

 from a marine to an aerial life involves much profounder changes 

 of environment, which have to be met by correspondingly im- 

 portant changes in the organism. This may be in part the 



1 SB. K. Akad. Wiss. Wien, 1889, p. 4, but the view is not universally 

 accepted. 



