VINK: notes on BRITISH EOCENE POLYZOA. 
157 
require certain genera of Mollusca, and only certain species of 
these are selected, dead shells and shell banks among the bivalves 
especially being in demand, Pecten opercular is and F. Genxirdii : 
Pectunculus, Oijtherea riidis, P kolas, Solen, TelUna crassa and ob- 
liqua, Fusus antiqiias, Nassa and CoUumhella. Other genera are less 
so, and of these Fissarella, Capulus, Buccinum, Purpura tetragona, 
Ostrea, and Cardiiim, are the chief Terebratuhc are good hunting 
grounds in the Coralline, but not in the Red Crag. With the ex- 
ception of Solen, Pecfunculus, and others, these genera are rare, or 
at least numerically so, individually in the Eocenes, and of these it 
may be said that very few examples are known in a worn or "dead" 
condition. Drifted shell banks are not common. Most of the species 
are in their native haunts, or where they have been removed, the 
genera, such as Cyprimi, are not those selected for attachment. 
The conditions of life again are not favourable : the Eocene of 
England consisting of either sharp sand or muddy clay, estuarine or 
fresh water beds. 
Sharp sand is also unfavourable for preservation, as in the case 
of the Oldhaven sand at Bromley, where the FectuncuU are in 
millions, with the surface nearly all decorticated, and in the London 
Clay, casts of the shells are alone preserved (save a few portions of 
the test) in apyritised condition. 
There is only one other reason I can suggest for their absence, 
i.e., that the Molluscoida had reached their apogee in the Cretaceous 
period, and only few genera and individuals represented this class of 
organisms, till other times and conditions more favourable to their 
existence came in, in other words, they were non-existent."" 
In the following monogTaph, for the sake of future students, I 
have collected together all that is known respecting the few Eocene 
species, but whenever I could I have corrected the older work by 
references to my own and the loaned collections ; the new species and 
varieties are additions to all previous lists, and described from ex- 
amples in my own cabinet. 
Brit. Assoc. Report on Recent Poljzoa, 1885-6, p. 192 of Report. 
