A MONOGRAPH 



OF THE 



FOSSIL ESTHERIA 



INTRODUCTION. 



Geologists, looking at fossils as witnesses of the varied conditions of land and water 

 in remote times, desire to inquire fully into the probable habits and relationships of every 

 organic relic of the past. Fossil shells, forming the chief portion of the materials in the 

 hands of the palaeontologist, become especially the subject of such inquiries, and are made 

 to yield evidence as to the relative age and the mode of formation of the several strata in 

 which they occur. It is by comparing the extinct shells with those now living, and 

 assuming for the fossil mollusc habits similar to those belonging to the most nearly allied 

 of its existing congeners, that geologists for the most part form a judgment as to the 

 character of many strata, whether they were marine or fluviatile in their origin, whether 

 formed in shallow or in deep water. We are not surprised that the evidence thus ob- 

 tained should often be weak and occasionally faulty, seeing that mere similarity in the 

 form of shells has sometimes to be taken as evidence of generic relationship or of specific 

 identity ; whereas the soft parts of the mollusc, now lost, might have borne other evi- 

 dence. 1 In nothing are naturalists so much deceived as by the manifold mimetic resem- 

 blances occurring throughout all kingdoms of nature. These are not wanting between 



1 A marked instance of palseontological uncertainty as to the relationships of certain bivalves occurs 

 in the case of some of the "Rhsetic" fossils, thus alluded to by Mr. Charles Moore, in the ' Quart. Journ. 

 Geol. Soc.,' vol. xvii, p. 502, when describing them under the generic name " Axinus, Sowerby:" — 

 " Few shells have been subject to greater transposition, or have been placed under so many different 



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