ON THE CAE GWTN CAVE. 121 



Macclesfield. It has gone north, while all the other shells still live 

 on our coast. 



Somewhat analogous is the occurrence of two locally extinct shells 

 in our postglacial river-gravels. Oorhicula fiaminalis and Unio 

 littoralis have gone to the Nile and Loire, while the Mammoth and 

 Tichorhine Rhinoceros, whose remains are found in the same 

 gravels, seem to indicate colder conditions, and the rest of the 

 freshwater Mollusca are still found in the Cam and other rivers of 

 the south-east of England. 



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 striae, which they refer to 

 glacial action. I exhibit * fragments of shell, picked up on the coast 

 of North Wales this year, which are similarly scored by the acci- 

 dents 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 Yale of Clwyd, or the equivalent beds elsewhere, and those 

 found in modern deposits of the same character on our coast at the 

 present day. In confirmation of which I exhibit* recent specimens 

 from 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 Jefeeys t says : " Our usual 

 form (which may be termed soUdula) 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 attenuata, in which the shell is smaller, more 

 compressed, and of a thinner consistency, is the Baltic form. 



In the var. truncata of Mactra solida the shell is thicker Sind the teeth 

 stronger. This form occurs " South of Devon and Cornwall, Tenby, 

 Irish coasts, Firth of Eorth, Clyde district, Orkneys, and Lerwick." 

 Mactra solida and the variety truncata have been chiefly noticed as 

 littoral and in southern latitudes, their furthest limit being Sicily, 

 where the former is also fossil; the only northern locality that appears 



* /. e. at the meeting of the Geological Society, 

 t British Oonchology, toI. ii. pp. 376, 377. 



