INTRODUCTION. XXXV 



Eocene associates of less questionable character, being 

 assuredly wholly distinct from any now living. Our Eocene 

 land and fresh- water shells (as well as the fresh- water forms 

 of previous epochs), although several, especially certani 

 Paludince and Planorbides, very closely, almost too closely, 

 approach existing types, are all regarded as distinct from 

 those that live upon our land now. It is a very remark- 

 able fact, tliat among them, as shown by Mr. Searles Wood 

 and Mr. Frederic Edwards, there are species which cannot 

 be separated from existing American forms. The true 

 source of our Molluscan fauna was first manifested by 

 the assemblage of testacea preserved in the deposit 

 called Coralline Crag, a formation at first regarded as Mio- 

 cene, but now held to be of older Pliocene age. In that 

 ancient sea-bed are to found many of the ancestors of our 

 living shell-fish, mostly of them forms which we regard as 

 southern types. Some of these seemed to have lived on 

 continuously to our own time, but the majority, after 

 struggling with the advent of less favourable conditions 

 during the deposition of the succeeding Red Crag, were 

 banished from our seas, when the frigid conditions of the 

 glacial epoch set in, and did not return until the restoration 

 of new ages of summer. With glacial conditions came 

 Arctic species, many of which linger still within our area. 

 It seems very probable that some of these are gradually 

 being extinguished, and that a few of our testacea, such as 

 Pecten Danicus, of which the number of dead specimens 

 taken is quite disproportioned to the rarity of living 

 examples, are close upon the time of their final extirpa- 

 tion, whilst others, such as Pecten Islandicus, and Leda 

 ohlonga^ may have been blotted from the lists of the living, 

 even since the occupation of the British Isles by man. 

 Our land and fresh-water testacea, althout:h we cannot 



