PRELIMINARY REMARKS. 3 



It has been observed by several geologists that " the triple arrangement becomes 

 considerably modified as we proceed towards the north of England and Scotland ; for in 

 Derbyshire and Yorkshire the true coal-bearing strata do not cease with the Millstone 

 grit, but are intercalated with it ; and in Yorkshire the Limestone (Yoredale rocks) contains 

 several coal seams, flagstones, ironstones, &c. ; and still further northwards, in Northum- 

 berland and the south of Scotland, the lower Limestone becomes frequently divided by 

 intervening beds of grit, shale, and coal." 1 



To ascertain the true stratigraphical limits of the group is therefore a matter of great 

 importance not only to the geologist but likewise to the palaeontologist, because the number 

 of species varies considerably in the different portions of the system ; and to offer at present 

 an example, without anticipating the conclusions we may arrive at ourselves after having 

 completed the review of the different species, we will borrow a few lines from p. 244 of 

 the 2d vol. of the ' Geology of Yorkshire,' in order to show what were the numerical 

 proportions arrived at in 1836 by its distinguished author: 



" Coal formation . . 3 species of Brachiopoda. 

 Millstone grit . 4 ,, „ 



Yoredale rocks . . 29 ,, „ 



Lower Scar Limestone .96 ,, „ 



" It appears to be in the upper part of the Lower Scar Limestone that the greatest 

 number of fossils of all kinds occur ; they grow continually less and less plentiful as we 

 ascend in the series of the Yoredale rocks, Millstone grit, and lower part of the Coal measures. 

 In the upper part of the Coal series all the species vanish." 



It was, therefore, during the deposition of the lower divisions of the Carboniferous 

 groups, i. e., of the Mountain Limestone and its accompanying shales and sandstones, that 

 the Brachiopoda flourished ; while in the upper divisions of the period are entombed the 

 remains of that stupendous vegetation which we all so much admire. 



The Brachiopoda of the Carboniferous period are therefore both numerous and widely 

 distributed, and have been partially the subject of several important works published at 

 various intervals both at home and abroad. Still of British fossils they seem to be those 

 perhaps the less generally or completely understood, and especially so if we are to infer 

 from the great extent of misnaming prevalent in almost every public and private collection, 

 not merely in this country but also on the Continent. This unfortunate state of things 

 seems in great measure attributable to the vague and unsatisfactory manner in which 

 certain species have been described and illustrated, as if a few words of description, taken 

 from some obscure fragment at times even imperfectly represented, were sufficient to 



Old Red Sandstone into the Carboniferous period ; but this is not the place for either discussing or en- 

 larging upon such matters, and we must content ourselves by referring the reader for further information 

 to the numerous and excellent geological works and memoirs by Conybeare and Phillips, Buckland, Sir 

 R. Murchison, De la Beche, Prestwich, De Verneuil, De Koninck, and others. 

 1 Tennant, 'A Stratigraphical List of British Fossils,' p. 90, 1847. 



