672 REPORT—1885. 
‘Eocene Polyzoa.’ 
Note on the Scarcity of Eocene Polyzoa. 
Considering the richness of other sections of organic life in the Eocene, 
the great poverty of the Molluscoida (Brachzopoda and Polyzoa) is very 
remarkable, and this poverty does not arise from an oversight on the 
part of collectors, or for want of looking for. Some thousands of spe- 
cimens having passed through the hands of one or other of us, it is 
possible to speak with some amount of certainty upon this head. Of the 
few species listed, Memb. Lacroixii, Flustra crassa, and Lunulites urceo- 
latus are the commonest, but are by no means abundant; and the first 
two have a wide range in Hocene time. Neither do they occur in any 
quantity in the Continental Hocene fauna, as an examination of some 
thousands of shells from the Paris basin will expose an equal sparseness. 
The cause of this absence is difficult to explain, as far as the free 
growing Cyclostomata are concerned, unless that the soil, or food, or other 
conditions of life, was not favourable for their development. For the 
adnate Cheilostomata, the want of such shells or adherent surfaces proper 
for their habitat may be sufficient cause. 
Reasoning from the rich Crag fauna, it would appear that Polyzoa 
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, the Pecten opercularis and Geradi, Pectunculus, 
Cytherea rudis, Pholas, Solen, Tellinas crassa and obliqua, Fusus antiquus, 
Nassa and Oolumbella. Other genera are less so, and of these Fissurella, 
Capulus, Buccinanops (?), Purpura, Tetragona, Ostrea, and Cardium are the 
chief. Terebratule are good hunting grounds in the Coralline, but not in 
the Red Crag. With the exception of Solen, Pectunculus, and others, these 
genera are rare, or at least numerically so individually in the EKocenes, 
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 Cyprina, are not those selected for attachment. 
The conditions of life again are not favourable ; the Eocene of Eng- 
land 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 Pectunculi are in millions, 
with the surfaces nearly all decorticated, and in the London clay. Casts 
of the shells are alone preserved (save a few portions of the test) in a 
pyritised condition. 
There is only one other reason I can suggest for their absence, 7.e. 
that the Molluscoida had reached their apogee in the cretaceous period, 
and only few genera and individuals represented this class of organism, 
till other times and conditions more favourable to their existence came 
in, in other words, that they were non-existent. 
Mr. Bell has certainly given very fair explanations of some of the 
causes which prevented the full development of a Polyzoan fauna, and I 
am glad to be able to give currency to his views. In my fifth British 
Association Report I remarked that we might owe the scarcity to want 
of research. This, however, seems not to be the case. The following 
is the only list that I can supply :— 
