PEEHAKENCE OF FEESH-WATEE FAUNAS. 209 



with certainty among the terrestrial pulmonates."" It must be pre- 

 mised that these remains constitute only an infinitesimal fraction of 

 the entire pulmonate fauna of the period, and therefore it is impos- 

 sible to say in what ratio the recent generic types stand to types 

 that are not recent, but which may have lived and flourished and 

 left no traces of their existence behind them. The case as it 

 stands, however, is sufficiently interesting, and permits us to as- 

 sume that the process of modification among the land MoUusca 

 was an exceedingly slow one, probably very much slower than 

 among the corresponding marine forms of life. This seems also 

 to have been the rule with the fresh-water MoUusca, whose devel- 

 opment in time runs about parallel with that of the terrestrial 

 Pulmonata. It is now practically certain that the range of the ge- 

 nus Anodonta extends at least as far back as the Devonian period," 

 and not improbably the forms described as Naiadites, from the 

 Coal-Measures of Nova Scotia, are true Unios. No unequivocal 

 fresh-water Mollusca are as yet known from the Permian forma- 

 tions, and even in the Trias the number of such forms, doubtfully 

 referred to the genera Unio and Myacites, is very limited. Only 

 with the succeeding formation do we have the first considerable 

 development ; but from that time onward to the present day the 

 number of species, in most cases referable to existing genera, rapidly 

 increases. The earliest fresh- water gasteropods date from the Juras- 

 sic period, and are comprised almost altogether in the modern genera 

 Neritina, Planorbis, Vivipara, Valvata, Hydrobia, and Melania. It 

 should here be observed that, while from the preceding data it may 

 appear that, with few exceptions, all the earlier (as well as later) 

 fluviatile mollusks, whether lamellibranchs or gasteropods, be- 

 longed to genera which still flourish in our fresh waters, this needs 

 not necessarily have been the case as a matter of fact ; for many 

 forms that, by reason of their association with marine organic types, 

 have in themselves been classed as marine, may have been of a dis- 

 tinctly fresh- water habit. It is inconceivable that the only evidences 

 of life in the ancient waters of the land should be centered in the 

 few organic remains that we recognise to be of an indisputably 

 fresh-water character. Rivers, then as now, discharged into the 

 sea, and deposited large quantities of sediment along the conti- 

 nental borders. It could scarcely have happened otherwise than 

 that more or less perfect parts of shells, swept down by the currents, 



