FAUNA OF THE CEAG. 
203 
depth of the sea and climate, between the periods of the 
Coralline and Red Crag, causing the fauna in each to differ 
far more widely than would appear from the above numer¬ 
ical results. 
The value of the analysis given in the above table of the 
shells of the Red and Coralline Crags is in no small degree 
enhanced by the fact that they were all either collected by 
Mr. Wood himself, or obtained by him direct from their dis¬ 
coverers, so that he was enabled in each case to test their 
authenticity, and as far as possible to avoid those errors 
which arise from confounding together shells belonging to 
the sea of a newer deposit, and those washed into it from a 
formation of older date. The danger of this confusion may 
be conceived when we remember that the number of species 
rejected from the Red Crag as derivative by Mr. Wood is no 
less than 25. Some geologists have held that on the same 
grounds it is necessary to exclude as spurious some of the 
species found in the Norwich Crag proper; but Mr. Wood 
does not entertain this view, believing that the spurious 
shells which have sometimes found their way into the lists 
of this crag have been introduced by want of care from 
strata of Red Crag. 
There can be no doubt, on the other hand, that concholo- 
gists have occasionally rejected from the Red and Norwich 
Crags, as derivative, shells which really belonged to the seas 
of those periods, because they were extinct or unknown as 
living, which in their eyes afforded sufficient ground for sus¬ 
pecting them to be intruders. The derivative origin of a 
species may sometimes be indicated by the extreme scarcity 
of the individuals, their color, and worn condition ; whereas 
an opposite conclusion may be arrived at by the integrity 
of the shells, especially when they are of delicate and ten¬ 
der structure, or their abundance, and, in the case of the 
lamellibranchiata, by their being held together by the liga¬ 
ment, which often happens when the shells have been so 
broken that little more than the hinges of the two valves 
are preserved. As to the univalves, I have seen from a pit 
of Red Crag, near Woodbridge, a large individual of the ex¬ 
tinct Valuta Lamherti^ seven inches in length, of which the 
lip, then perfect, had in former stages of its growth been 
frequently broken, and as often repaired. It had evidently 
lived in the sea of the Red Crag, where it had been exposed 
to rough usage, and sustained injuries like those which the 
reversed whelk, Trophon antiquum^ so characteristicf of the 
same formation, often exhibits. Additional proofs, howev¬ 
er, have lately been obtained by Mr. Searles Wood that this 
