124 D. G. Davies — The Millstone Grit of North Wales. 



at its base, and downwards to the base of the Carboniferous Lime- 

 stone, thus affording a good illustration of a species surviving during 

 an immense period, amid widely diverse circumstances, and changes 

 in the condition of the sea-bed. Among these shells at Sweeney 

 are found fragments of Orthoceratites (see Mr. Prosser's paper, 

 already referred to). One tolerably perfect specimen, measured 6ft. 

 long by 1ft. Sin. across its mouth, Some idea may be formed of the 

 abundance of smaller life in the seas of those days by considering 

 what havoc must have been made by a creature like this. I call 

 attention to this fossil for another reason. Treflach, a place close by, 

 is a locality distinguished for the great size of the Cephalopoda in 

 the Mountain Limestone. Both Nautili and Ortlioceratites will, for 

 size, probably surpass those from any other place in England. So 

 that we have a small area which was inhabited by huge Cephalopods 

 during a period necessary for the deposition of strata 350 feet in 

 thickness, and we may infer, from the continued size of the in- 

 dividuals to the close of the deposit at this spot, that there was no 

 lack of food for the family during the whole of that time. There is 

 however a marked absence of fossils from the middle beds. This 

 absence arises no doubt from the fact that the favourite resorts ^ of 

 the organisms have not been touched by the quarryman. I have, 

 however, numerous fossils found in fragments obtained from the 

 neighbouring drift, which have evidently come from these middle 

 beds. Prom one of these have been derived numerous specimens of 

 ScMzodus, which appear here for the first time, being better known 

 in the later Permian beds. Mr. Davidson informs me that the 

 Ailiyris I have before mentioned as occurring in the uppermost beds, 

 is different from any of those found' in the Carboniferous Limestone, 

 and it is possible that here we may have an example of a species 

 which has changed during a long lapse of time, and has, in conse- 

 quence, been enabled to outlive all its congeners. Above the eight- 

 feet bed at Sweeney there are well defined trails of worms of con- 

 siderable size, and these are associated with numerous fucoidal 

 remains. The same grouping is seen in similar beds near Chork 

 Castle, nine miles to the north. Eeef-making corals, such as 

 Lithostrotion and Cyailiopliyllum, do not appear in the Sandstone 

 beds, but in the limestones of the Mold section, of equivalent age to 

 the beds below the eight feet bed at Sweeney, they occur in 

 abundance, illustrating the way in which genera, disappearing from 

 one locality through unfavourable conditions, continue to survive in 

 others under more favourable auspices ; and suggesting also that, as 

 the successive formation are dove-tailed, so to speak, one into the 

 other, rather than divided by sharp lines of separation, it is not wise 

 to speak dogmatically of the age of certain epochs by fossil evidence 

 alone. 



It will be seen, from the foregoing remarks, that the fauna of the 

 Millstone Grit bears a decidedly Carboniferous Limestone facies, 

 but above the middle beds numerous remains of Coal Measure plants 



' Or, more probably, the grave of tbe Cephalopoda ? — Edit. 



