INTRODUCTION. 5 



Carboniferous Limestone Series of Scotland when there is a frequent alternation of 

 beds with a typically marine fauna with those containing Carbonicola and its 

 congeners. 



The Penneystone Limestone of Coalbrookdale, in which occur typically marine 

 shells — Edmondia, Sanguinolites, Schizodus, and Brachiopoda, together with 

 Anthracosia and Naiadites (the " Unios " of Professor Prestwich), would appear to 

 be an exception to this view. There is, however, evidence to show that the 

 Unio-like shells contained in this probably mainly marine bed had been washed 

 down into it, on account of the occurrence of typical land plants in the bed. The 

 marine fauna, too, is very interesting and characteristic, containing a much larger 

 percentage of Lamellibranchiata than obtains in the Carboniferous Limestone as 

 a rule, and probably indicating very shallow water and a littoral deposit. The 

 marine fauna closely resembles that found in the Redesdale ironstone of 

 Northumberland, with the exception that the latter does not, so far as I can learn, 

 contain the Unio-like forms; in the "Chance Penneystone" of Coalbrookdale, 

 where marine forms again appear, Carbonicola, &c, are conspicuously absent ; and 

 conversely, at the other horizons at which these shells are found, marine forms 

 do not occur. 



This view was held by Prof. Prestwich, who, in his memoir on the ' Geology 

 of Coalbrookdale,' 1 says, "We find .... evidences of strong river action in the 

 presence of transported vegetables .... and fluviatile shells, intermingled with 

 the marine Testacea of the sea into which they were drifted." And again, in a 

 note at p. 466, referring to the mixture of marine and fluviatile forms, and the 

 question of their identification by the presence or absence of eroded beaks, " I 

 have examined a considerable suite of shells apparently belonging to this genus 

 (Unio), but, as they are all casts in ironstone, I have not been able to come to 

 any very decided opinion ; nevertheless the greater number of specimens 

 exhibited no trace of erosion, but a few, and amongst them several from the 

 Penneystone, decidedly did," — an observation which I shall illustrate in shells 

 from the North Staffordshire Coal-field. 



Speaking of the probable condition under which the Coal-measures of 

 Coalbrookdale were deposited, Sir R. I. Murchison says : 2 



" Doubtless, therefore, as hinted at in the preceding chapter, this tract of 

 Coalbrookdale must originally have been a bay of the sea, into which streams of 

 fresh water discharged materials derived from those lands, the contiguity of which 

 has been previously inferred from the existence of fresh-water limestone in the 

 adjacent coal-fields. This view is also quite in accordance with that of Mr. 

 Prestwich, who is of opinion ' that the alternations of fresh-water shells with 



1 ' Trans. Geol. Soc. Lond.,' ser. 2, vol. v, pt. 3, 1840, p. 469. 



2 ' Silurian Syst.,' p. 105. 



