INTRODUCTION. 9 



proximity of land. The shells of Naiadites (Anthracoptera) are very large and fine ; 

 and whatever conditions obtained, they were very favorable to the growth and life 

 of the species. As to Beith, the other locality where this shell occurs, Mr. Robert 

 Craig writes me that it is a rare shell there except in two places, where it existed 

 in large colonies, at Roughwood in a shale averaging three feet in thickness, which 

 rests on a bed of coal and is very fossiliferous. The lowest two inches contain a 

 mixture of marine and brackish water forms, above which the fossils are marine. 

 In the Beith quarries Myalina is found in a shale ten inches thick, which rests on 

 a coal, and is associated with fish remains, of brackish water origin. The real 

 nature of these beds is therefore quite open to question. In the brackish or 

 possibly fresh-water beds of the Cement-stone Group occurs a slighter form, which 

 Mr. Robert Btheridge, jun., describes * as of slighter make than those from Cults ; 

 and he refers it to the form described by Captain Thomas Brown as Avicula 

 modioliformis ; 2 but he says " I cannot distinguish any characters, either external 

 or internal, by which specifically to distinguish Captain Brown's shell from 

 Dr. Fleming's, and I propose to adopt the name given by the former as a varietal 

 designation for the thinner and lighter form of Myalina crassa." 



It is quite possible that the change in size was merely due to change in environ- 

 ment, and that we have here an example of a species, originally marine in habitat, 

 growing luxuriantly under marine littoral conditions, but dwindling rapidly in 

 size, and passing off into varietal forms, under the fresh-water conditions of the 

 Middle and Upper Coal-measures. 



The universal absence of marine forms of life in the Middle and Upper Coal 

 stages ¥ and G throughout the coal-fields of Europe is very striking, and 

 Continental geologists are almost unanimous in considering Garbonicola, Anthra- 

 comya, and Naiadites to have had a fresh-water habitat. Doubt has been thrown 

 on this point by Salter, 3 who says, " Meantime there is yet more doubt if they 

 can be fresh-water forms, associated as they always are with Modiola and other 

 true sea animals," — a statement certainly not borne out by observation. The 

 association with true marine animals is certainly not " always," and the word 

 " rarely " might have been substituted for it with greater approach to truth. But 

 I shall show later on that Salter was led to misinterpret and neglect several 

 previously observed and obvious anatomical facts in order to establish Anthracosia 

 and his new genus Anthracomya as marine forms. Prof. Hull 4 states, "These 

 bivalves (the Anthracosia of King) are the bane of palaeontologists, and after a 

 long consideration of their relationship to other fossils in Carboniferous strata, 



1 ' Quart. Journ. Geol. Soc. Lond.,' vol. xxxiv, p. 13, 1878. 



2 ' Eoss. Conch.,' 1849, p. 162, pi. lxvi, fig. 19. 



3 ' Geol. Survey Mem.,' "Iron Ores, South Wales," pt. iii, p. 227, 18(51. 

 1 Supra cit., ' Quart. Journ. Geol. Soc.,' vol. xxxiii, p. 643. 



2 



