THE SHELL -LIMESTONE OF DURHAM. 123 



traca found in the sliell-limpstone of the Permian formation of 

 Durham, though in some instances materials from the upper 

 limestone of Byers' Quarry have been used in working out 

 specific characters. 



The shell or fossiliferous limestone occupies a central position 

 in the calcareous beds of the series. It rests upon the compact 

 limestone, and is overlaid by the cellular or brecciated limestone ?, 

 which, in its turn, is superimposed by the crystalline, botryoidal, 

 and earthy, oolitic limestones, which are often grouped as the 

 upper or higher members. Entomostraca have only been found 

 in the shell and crystalline limestones in Durham. In Germany, 

 they occur in the Unter Zechstein, which is the equivalent of the 

 compact limestone, both of which have the marl-slate or kupfer- 

 schiefer for their base. From this it appears that Entomostraca 

 occupied a place in the marine fauna of Western Europe from 

 the commencement of the deposition of the Permian limestones, 

 until the last beds of the same were being accumulated. At 

 the same time it is evident that their geographical distribution 

 • was not general; for in numerous fossiliferous localities, both in 

 Durham and Germany, their remains are absent. In the shell- 

 limestone, the only locality where they have occurred in any de- 

 gree of abundance is at Tunstall Hill, and there their distri- 

 bution is very local. Occasional specimens have also occurred 

 in shell-limestone at Humbleton Hill, and the new Poor House, 

 Bishopwearmouth. In the higher members, their remains have 

 only been found in the crystalline limestone at Byers' Quarry. 

 Further researches may probably discover other localities where 

 they occur, though it is more than likely that their distribution 

 will always be limited to particular spots. 



In the limestone of Tunstall Hill, there occur irregular shaped 

 cavities filled with brown or yellow calcareous dust. In some, 

 the contents are simply dust ; in others, there are groups of 

 finely preserved fossils intermixed. These cavities, and their con- 

 tents were first noticed by Mr. Howse, who also observed them 

 in the shell-limestone of Dalton-le-Dale.* It was in one of these 



* " Catalogue of Fossils of the Perm. Syst. of Northumb. and Durham," p. 9. By an unfor- 

 tunate oversight, Silksworth was substituted for Dalton-lc-Dale, in the publication of this 

 paper in the " Annals of Natural History." 



