84 Proceedings of the Royal Physical Society. 



Craig's researches, about thirty genera and more than fifty 

 species have been met with.* 



In an important paper, " On the First Appearance of Cer- 

 tain Fossils in the Carboniferous Strata around Beith and 

 Dalry/'f Mr Craig has shown that a regular gradation takes 

 place in the appearance of species in the rocks of this district 

 which must be due to some physical causes. The number of 

 Mollusca met with in this basin may be accepted as a fair 

 illustration of that of the Lower Limestone Group generally ; 

 the numbers given by Mr Craig are as follows : — 



Turning to the Upper Limestone Group we may take as 

 examples the two localities which have been most thoroughly 

 investigated — Eobroyston and Orchard. Through the pains- 

 taking industry of Messrs Young and Armstrong these two 

 celebrated sj^ots have been thoroughly investigated. The 

 limestones are sparingly fossiliferous as at many other places, 

 but the overlying shales are surprisingly rich. The following 

 are the numbers of each class obtained by Messrs Young and 

 Armstronof : — 



The fossiliferous shale at both localities is particularly rich in 

 Mollusca of the first three classes mentioned rather than 

 other fossils. 



* " Sketch of the Carboniferous Basin of Dairy, Aj'rsh ire " (Trans. Geol. 

 Soc., Glasgow, 1869, iii., pt. 2, p. 271). 



t Ibid., 1875, v., pt. 1, pp. 36-50. 



X "Notes on the Fossils of the Orchard Limestone Series" (Trans. Geol. 

 Soc, Glasgow, 1877, v., pt. 2, p. 250). 



§ "The Fossils of the Cail)oniferous Strata of the W. of Scotland " {H)i(J.„ 

 1874, iv., pt. 3, p. 267). 



