DISTRIBUTION. 175 



" Yoredale series " has been used to denote two dissimilar lithological series, each 

 characterised by a different fauna. 



The Yoredale Series of Wensleydale and the north of England represents the 

 upper part of the Carboniferous Limestone, split up by wedges of detrital sands 

 and muds, which came in from the north. The fauna of the true Yoredale Series 

 is identical with that of the upper part of the Carboniferous Limestone, and differs 

 essentially from that contained in the Pendleside Series of the Midlands. In 

 reality, the Pendleside Series is homotaxially superior in position to the Yoredale 

 Series. I regard the following Lamellibranchs as typical of the Pendleside Series 

 and some of the succeeding beds : 



Chxnocardiola Footii, Baily, sp. Leiopteria longirostris, Hind. 



Posidoniella Isevis, Brown, sp. Pterinopecten papyraceus, Sow., sp. 



,, Kirlcmani, Brown, sp. ,, carbonarius, Hind. 



,, minor, Brown, sp. Pseudamusium fibrillosum, Salter, sp. 



,, variabilis, Brown, sp. Aviculopecten Losseni, v. Koenen, sp. 



Posidonomya Becheri, Bronn. ,, (jentilis, Sow., sp. 



,, membranacea, M'Coy. 



A very important factor in the distribution of Carboniferous Lamellibranchs is 

 the nature of the deposit, whether it is detrital or organic in origin. The fauna of 

 the limestones differs very considerably from that of the shales and grits. The 

 Nucdidai invariably are found in detrital deposits. 



In the Midlands the fossils of the Carboniferous Limestone occur very locally at 

 or very near the top of the series. Very little, indeed, is known of the distribution 

 of the fossils throughout the thick mass of limestone. When, however, this mass 

 splits up into beds and becomes the Yoredale Series, the Limestones and the Shales 

 between them can be examined individually, and a certain amount of detailed 

 knowledge exists. In Derbyshire, Staffordshire, and South-west Yorkshire fossils 

 occur plentifully in certain localities, a fact that may be noted in referring to the 

 localities given for each species, and to page 214. At these places, Thorpe Cloud, 

 Park Hill, Hill Bolton, and Settle, the upper beds of the limestone are almost 

 made up of fossils. I consider these localities to have been shell banks, for shells 

 of all sorts of habitats are present, the larger Gasteropods and Cephalopods full of 

 smaller specimens, the Lamellibranchs as a rule with both valves misplaced or 

 detached. The remains of animals which had a different habitat are all crowded 

 together in profusion. Many specimens indicate slight attrition due to rolling. 

 The Brachiopods, on the other hand, found in profusion amongst the other shells, 

 probably lived on these banks, in colonies, for they are nearly always found, with 

 both valves in position, though, of course, this may be due to the fact that the 

 valves were more firmly attached to each other than obtained in the ease of the' 

 Lamellibranchs. 



Many of the Lainellibranch species had a very long existence in Carboniferous 



