ORIGIN OF FRESH-WATER BIVALVES 



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

 a distance of 1600 miles from the sea (Fig. 9). Fholas rivicola 

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

 miles from its mouth. Cyrena, Corhicula, and prohal)ly Si^haerium 

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

 the exclusively marine Veneridae ; Potamomya (rivers of S. 

 America), and Himella (E. Amazon) are forms of CorhuJa. 

 The Caspian genera derived from Cardium (Adacna, Didacna, 

 Monodacnct), have already been referred to. Nausitora is a 

 form of Teredo, which lives in fresh water in Bengal. Raiyjia, 

 Fischeria, and Galatea probably share the derivation of the 

 Cyrenidae, while in Iphigenia we have one of the Donacidae 

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

 estuarine life. The familiar Scrobicularia piperata of our own 

 estuaries is a TeUina, which lives by preference in brackish 

 water. 



The great family of the Unionidae is regarded by Neumayr ' 

 as derived from Trigonia, the points (jf 

 similarity l)eing the development (jf a 

 nacreous shell, the presence of a strung 

 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 I'elecypoda 

 are comparatively few in number, and 

 their origin is far more clearly discernible 

 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 l)oth obtain their 

 nutriment from matter contained in water. Similar remarks 

 ap})ly to fresh-water operculate Ciasterop(xla. But the passao-e 

 from a marine to an aerial life involves much profomider changes 

 <if environment, wliich have to be met l)y correspondinglv im- 

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



^ SB. K. Akad. ITiss. IVicn, 1889, p. 4, but the view i« not uiiivcrsallv 

 accepted. 



Fig. 10. — Trigonia ^jec- 

 (inata Laru. , Sj'duey, 

 N.S.W\ 



