10 KOCENE MOLLUSCA. 
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 Paleontographer is to give good figures and copious descriptions, in 
the hope to secure similar 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 mea 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 J 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 Palsontology, 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 Paleontologists, 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 Bivaives 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, Cucullea, Cardilia, 
