144 
WEYBOURN CRAa. 
comparatively few individuals, the discrepancy is probably in 
reality somewhat less. The proportion of Arctic to Mediterranean 
forms is— 
— 
Total. 
Arctic. 
Mediterranean. 
Weybourn Crag 
53 
9 
0 
Chillesford Crag - 
90 
7 
2 
Fluvio-marine Crag 
112 
9 
7 
Two Arctic species and two characteristic Arctic varieties 
appear for the first time in the Weybourn Crag, viz., Astarte ere- 
hricostata, Astarte elliptica, Astarte borealis^ oval var., and Saxienva 
arctica, gigantic var.—all four, however, being rare. Thus in 
the Weybourn Crag, in a list containing only half the number of 
species, we find as many Arctic forms as in the Norwich Crag, and 
more than have occurred in the Chillesford Crag. In the number 
of individuals also the Arctic species are decidedly more plentiful, 
and hitherto no species having an exclusively southern range has 
been found in the newer deposit.* 
There still remains one shell to which especial attention must 
be drawn ; for Tellina halthica (Fig. 33, p. 138) appears for the 
first time in the Weybourn Crag, and seems to have taken pos¬ 
session of these seas immediately after its arrival. Thus it is 
usually by far the most plentiful shell on this horizon, and in 
many places it outnumbers all the other species put together. 
To anyone not intimately acquainted with East Anglian geology 
the stress laid by Messrs. Wood and Harmerf on the sudden 
appearance of this mollusc may appear to be absurdly exaggerated. 
But though I cannot follow them in regarding its incoming as 
marking a break between the Crag and the Lower Glacial 
de[)Osits—for the species is not an Arctic one, and the associated 
fauna is characteristically that of the Crag—yet the arrival of a 
prolific species like this is important, and quite sufficient to out¬ 
weigh a large number of records of rare shells, the finding of 
which depends mainly on the amount of time spent in the search. 
Prof. Prestwich has attempted to account for the abundance of 
Tellina. halthica in the Crag of the north-east portion of Norfolk, 
and its entire absence in the whole of the rest of the Norwich 
Crag, by pointing to the occasional absence of this species from 
bays on the present coast.{ In the case of these newer Crags, 
however, there is no possibility of any separation of the two areas 
by a barrier, and if the Weybourn Crag and Norwich Crag are 
contemporaneous, we must be dealing with two closely adjacent 
* I agree with Mr. Edgar Smith that Astarte incrassata of the “ Cromer Memoir ” 
is a wrong determination. The shell is Astarte elliptica, of which species other 
specimens have since been obtained. 
f See Introduction to the Crag Mollusca; Palaeontographical Soc. ; and various 
papers. 
t Quart. Journ. Geol. Soc., vol. xxvii., pp. 471, 472. (1872.) 
