CORALLINE CRAG. 
37 
were there, but the Bryozoa had grown on a firm base and could 
not be detached. The evidence of the Bryozoa themselves is 
found to point in the same direction; for an abundant Bryozoan 
fauna, such as we find in the Coralline Crag, can only flourish in 
strong currents, and where the water is clear. 
The fauna of the Coralline Crag is so rich as to afford abundant 
evidence for deciding under what conditions the beds were formed, 
and for making a comparison with other deposits. One pecu¬ 
liarity of the deposit, however, must not be lost sight of; calca¬ 
reous strata of this character are exceptional among the European 
Pliocene groups, and it is not easy to find anything closely similar 
in the present seas of Europe. Such unusual conditions of deposit 
as those in which these calcareous formations were accumulated 
must, of course, encourage the growth of certain forms of life, but 
act inimically on others, and it is necessary to make allowance for 
this effect when we compare the fauna of the Coralline Crag with 
the fauna of strata of the Diestian type, or with that of the 
clayey deposits of St. Erth. 
Taking the different classes of fossils in the order of their 
relative importance in the formation, the first place must be given 
to the Mollusca, though in many parts of the Coralline Crag 
Bryozoa are at least equally abundant. The number of species of 
mollusca recognised by Wood as belonging to the Coralline Crag 
amounted in 1882 to 420, of which he considered that 251, or 
60 per cent., are still living. Jeffreys, on the other hand, in 1871 
made the total 316, of which 84 per cent, belong to recent forms. 
Even if we leave out of account the rare extinct species described 
by Wood after Jeffreys published his criticisms, the discrepancy 
is still nearly as great, though both workers were using practi¬ 
cally the same material—the Wood collection in the British 
Museum. This extraordinary difference is doubtless due, on the 
one hand, to the tendency of Jeffreys to refer characteristic fossil 
forms to living species, and on the other hand, to Wood^s habit 
of considering any well-marked and constant variation in a Crag 
fossil from its nearest living allies to be sufficient to constitute it 
a new species. It is the old question—What is a species ?—for 
there is seldom any discrepancy between these two authors as to 
the close alliance of the recent and fossil mollusca identified by 
the one but kept apart by the . other. 
The discordance in the percentage of extinct forms in the 
Coralline Crag when the mollusca are examined by different 
conchologists, shows to what an extent the ‘‘ personal equation ” 
comes into play, and how dangerous is the use of the per-centage 
test for correlating different deposits, unless where all the fossils 
have been determined by the same person. 
In preparing this Memoir, and especially while studying the 
unknown fauna of Lenham, it has been necessary to make use of a 
number of monographs on French, Belgian, German, and Italian 
Pliocene mollusca, and it has seemed to the present writer that, 
comparing these monographs with the work of Wood, there is an 
equal or even greater tendency in most of the authors to accept 
