10 EOCENE MOLLUSC A. 



and critically examined by their successors, we are so much nearer a successful decision 

 only by a more general concurrence of opinion in favour of one set of determinations than 

 of the other. 



The duty of the Palaeontographer is to give good figures and copious descriptions, in 

 the hope to secure simitar determinations by the largest number of those naturalists 

 who have well studied the intricacies of the subject, and thus to afford a test by which 

 those who succeed may gauge and determine the value of his conclusions. 



The difficulty which, in the course of my study of the Mollusca, I have frequently 

 encountered in assigning the true specific value to forms which, in a series of individuals, 

 exhibited such an approximation to other forms called specifically distinct, long since raised 

 in me a doubt of the reality of specific distinctions as a fact in nature, which reflection upon 

 the general nature of organisms matured into a conviction that all organisms originated by a 

 natural process of slight variation accumulating in a given direction out of other and pre- 

 existing organisms. I was therefore, fully prepared for the enunciation of the theory of 

 Mr. Darwin, that all forms have originated by selection, and I readily concede that process 

 to be one of the most powerful, if indeed it be not the sole cause of all the varied forms of 

 being that have peopled the earth. Nevertheless, specific distinctions, empirical or artificial 

 though they may be, must always be to a certain extent recognised as essential to the 

 proper working out of our knowledge of Palaeontology, and particularly to a correct appre- 

 hension of the true ages of geological formations, and of the reduction of those widely 

 severed in area to a common horizon. The general recognition, however, of such an 

 origin for organisms, if indeed that ever be conceded by reluctant Palaeontologists, will, 

 in addition to the importance of the discovery of so great a truth, be no little boon to the 

 hard-working naturalist, whose labours have been seriously aggravated by a desire for 

 species making. 



It is intended here to describe all the species of British Bivalves belonging to the 

 Eocene or older Tertiaries of England ; these, with the Crag and overlying deposits, con- 

 stitute the Tertiary remains in this country. 



The great Eocene Formation in England has been separated into ten distinct series of 

 deposits, viz., the Bembridge, Osborne, Headon, Barton, Bracklesham and Bagshot series, 

 the London Clay, the basement bed of the London Clay, the Reading and Woolwich series, 

 and the Thanet Sands. These divisions are based principally upon geological and 

 lithological evidence. I have not been able to characterise these various distinctions by 

 their organic contents, but have merely introduced the localities of the different species so 

 far as they are known to me. 



The Marine Fauna of the Eocene Deposits appear to have their connexion with the 

 existing types of the eastern seas, where several of the Eocene genera are only now to be 

 found ; this is rather more strongly displayed by the Cephalopoda and Gasteropoda than 

 by the Bivalves, although in this latter division we have not less than seven genera now 

 confined to seas lying south-east of this country, viz., Vulsella, Cucullaa, Cardilia, 



