•204 MR ROBERT KIDSTON ON 



specimens in treating of this species, though the Radstock Series of the Upper Coal 

 Measures, Somerset, have yielded me my largest and finest barren specimens. 



It is not necessary here to enter into the geology of the Yorkshire Coal Field. 

 This has been fully done in the Geology of the Yorkshire Coal Field* and in other 

 works dealing with this subject. It may be simply noted that probably all the divi- 

 sions of the Coal Measures are present in this Coal Field, — the Upper, the Middle, and 

 the Lower Coal Measures, but the Upper Coal Measures are only represented by " Red 

 Beds" from which I have not yet seen any specimens, though I believe some plant 

 remains have been found in them at Conisborough Pottery. t 



The Middle and Lower Coal Measures contain all the workable seams in this Coal 

 Field, but the great coal-yielding series of the Yorkshire Coal Field is the Middle Coal 

 Measures. 



The Coal Measures are largely worked in that portion of the county which lies 

 around Halifax, Bradford, and Leeds, and which extends southwards to the neighbour- 

 hood of Sheffield. 



In 1886 \ I united Dactylotheca {Pecopteris) dentata, Brongt., with Dactylotheca 

 [Pecopteris] plumosa, Artis, sp., while preparing the Catalogue of the Palaeozoic Plants 

 in the British Museum, and I firmly held this opinion till about three years ago, when 

 some specimens submitted to me from Yorkshire led me to believe that Pecopteru 

 dentata, Brongt., was specifically distinct from Pecopteris plumosa, Artis, sp. § 



This latter opinion I saw, very shortly after, full cause to reject ; and the points con- 

 nected with the fructification, on which I thought the species might be separated, and 

 to which I shall more fully refer, were found to be entirely dependent on the position 

 of the fruiting portions on the frond and their state or condition of development. 



On the three plates accompanying this paper, figures are given of the typical plant 

 as well as of a number of forms of Dactylotheca plumosa, Artis, sp., to which specific 

 names have in some cases been given. It is an extremely variable species, — the extreme 

 forms differing so much in appearance that they have given rise to the creation of 

 several supposed species, all of which, when one has the opportunity of studying a large 

 series of specimens, are shown to pass into each other by insensible gradations, and which 

 seem to represent only different portions of what must have been a very large frond. 



I have therefore found it quite impossible to draw any line of demarcation between 

 what might appear at first sight such distinct forms as Sphenopteris crenata, L. and H., 

 on the one hand, and Pecopteris dentata, Brongt., on the other. In fact, these differences 

 seem to depend in great measure on whether the fragment is barren or fruiting, and on 

 the position it held on the frond of which it originally formed a part. 



Several of the species here placed under the name q{ Dactylotheca plumosa, Artis, sp., 



* Memoirs of the Geological Survey of England and Wales. By A. H. Green, E. Russell, &c. London, 1878. 

 t See Kidston, "On the Various Divisions of British Caihoniferous Rocks as determined by their Fossil Flora, 

 J'roc. Roy. Phys. Soc. Edvn., vol. xii. p. 210, 1894. 

 \ Oatal. Palceoz. Plants, p. 128. 

 § Trans. York Nat. Union, part xviii. p. 106, 1803. 



