INTRODUCTION. 7 



LiMNETis Wahlbergti, ZovSn. Ofvers. k. Vet. Freshwater marshes, Natal (J. Wahlberg). 

 Akad. Forh,, Arg. 3, No. 2, p. 57, 

 1847. 

 — GouLDTi, Baird. Fresh water, at St. Ann's, twenty miles from Mon- 



treal (C. Gould). ^ 



The recent Estherm are found in fresh water, rarely in brackish water. Guided by 

 this fact, and taking for granted that our fossils were true Estherice, and that Estheria 

 always have had freshwater habitats, we should suppose that the deposits in which these 

 fossils are found, free from any appearance of having been drifted, must have been formed 

 in rivers, lakes, or lagoons. Applying, however, the same rules in judging of the nature 

 of the fossil molluscs and other organic remains that occasionally accompany some 

 of these Estherice, we must regard the Lingula of the Old Red (of Livonia), the Spirorbis, 

 the Avicula, the Anthracosia, and AnthracomyfB'^ of the Carboniferous shales, and the 

 Lingula and Pleurophorus of the Trias, as truly marine shells. Many, however, of our 

 fossil Estheria occur in strata destitute of any such evidence of marine conditions ; and 

 possibly the occasional mixture of the marine and freshwater organisms may have been 

 the result of driftage (the free- swimming EstherifzhQmg readily swept away by a flood), or 

 of very rapid changes of condition, such as might be brought about by the alternate occupation 

 of a lagoon by sea- and river- water.^ Seeing, too, that the recent Estheria appear, as it 

 were, suddenly (like the Apus) in pools and ditches of rain-water, and are quickly deve- 

 loped in tanks and ponds dry for even ten or eleven months in the year, it is not unlikely 

 that pools of fresh water, temporarily formed on a flat seashore, may have been inhabited 

 by Estheria, destined to be quickly buried in the first wind-drift of sand, or at the return 

 of high tides. As an inhabitant of brackish water, the Estheria would be still more likely 

 to have been occasionally accompanied by marine shells : nor can we say that the fossil 

 associates quoted above were not inhabitants of brackish water, or of salt lakes; for 

 experience is the only guide to the naturahst in determining whether the members of 

 many of the moUuscan groups afiect marine, brackish, or freshwater habitats. 



Perhaps some might like to think that at first marine conditions alone suited aquico- 

 lous animals, and that some have subsequently taken to brackish and freshwater habitats ; 

 and this may have been the case with Estheria : but, except for the " progressive " aspect 

 of the argument, the converse might just as well hold good for the Lingula,^ SpirorUs, 

 Avicula, AntJiracosia, Anthracomga, and Pleurophorus, mentioned as being found in the 

 older rocks in company with Estheria. 



Of the living moUuscan genera that are known to have fluvicolous as well as marine 



1 Dr. Baird has kindly assisted me in drawing up this table of the recent Estherice and their allies. 



2 According to Mr. Salter, ' Mem. Geol. Survey, 1861, Iron-ores of South Wales,' &c. 



3 See Sir C. Lyell's observations on the value of Spirorbis (in the fossil state), and barnacles (recent) 

 in certain cases, as evidences of the occasional inroad of salt water into swamps, killing the marsh-plants 

 and leaving behind such shells as the above, as well as Modiolce, &c. (' Notices of the Royal Institution of 

 Great Britain,' vol. i, p. 285.) 



* See further on, for remarks on the Lingula tenuissima of the Trias. 



