180 CAVES AND CAVE DEPOSITS. 
All these are recorded by Gwyn Jeffreys as now occurring on 
the coast of the British Isles, except Astarte borealis, of which 
only dead and, possibly, derived shells have been found. All 
except Pecten varius and Fissurella greca have been found in the 
marine drift of St. Asaph and Colwyn ; and the two exceptions 
go for nothing, as these shells are common on our coast at the 
present day. 
There is only the As/arte borealis which is locally extinct, and 
it occurs in the high and low marine terraces, from Moel Tryfan 
to Macclesfield. It has gone north, while all the other shells 
still live on our coast. 
In deposits of such antiquity we might expect to find some 
locally extinct forms; but no one who compares the shells 
found in the drift outside Cae Gwyn Cave with those of any of 
the undoubted glacial deposits, such as that at Bridlington, 
could allow that the Cae Gwyn shells indicate glacial conditions. 
Some have seen on the shells in the St. Asaph Drift, outside 
the Cae Gwyn Cave, and elsewhere, small striz, which they 
refer to glacial action. I have, however, picked up on the coast 
of North Wales this year fragments of shell which are similarly 
scored by the accidents of a gravel beach. Some are from 
Deganwy, some from the Menai Straits, all too far from any 
shell-bearing drift to have been derived from it. 
It has been remarked that the shells in these marine drifts, 
though nearly all of existing species, are thicker than those now 
living on our coasts. It is natural that the thicker shells and 
the thicker parts of shells should have the best chance of being 
preserved among the stones and sand of a sea-beach; but I 
have failed to see any difference in this respect between the 
shells in the marine drift of the Vale of Clwyd, or the equivalent 
beds elsewhere, and those found in modern deposits of the same 
character on our Coast at the present day. I have found recent 
specimens on the coast of North Wales or further south, quite 
as thick as, or rather, I should say, much thicker than any of 
those in the marine drift. 
In many cases the southern varieties are characterized by their 
thickness and the northern by their thinness, as, for example, in 
the case of Tellina Balthica, of which Gwyn Jeffreys says: 
‘*Our usual form (which may be termed so//du/a) abounds in all 
the Tertiary deposits, including the boulder-clay or ‘till’ and 
the Mammalian Crag. It may, therefore, be regarded in the 
main as a northern species; but it is likewise common in many 
parts of the south of Europe.” The variety a¢/enuata, in which 
the shell is smaller, more compressed, and of a ¢hinner con- 
sistency, is the Baltic form. 
In the var. /runcata of Mactra solida the shell is ¢hzcker and the 
teeth s/ronger. This form occurs “South of Devon and Cornwall, 
Tenby, Irish Coasts, Firth of Forth, Clyde district, Orkneys, 
and Lerwick.” JMactra solida and the variety /runcaéa have been 
