72 



POST-TERTIARY ENTOMOSTRACA. 



and a quarter of a mile in breadth, and the clay is at its extreme point twenty-five feet 

 deep. 



3. The lower part of the clay is distinctly laminated, as in the Paisley and other 

 beds, and is far more sandy than the upper clay, becoming, indeed, the workmen informed 

 us, a sand bed at its deepest point. 



4. The trough in which the fossiliferous clay is packed is a hollow in the Boulder 

 Clay, which crops out on neighbouring elevations. 



The shells are met with at all points from the bottom to near the surface, but are 

 most abundant (as in the Paisley beds) in the lower part of the clay. 



Nothing can be more striking than the general physical conformity between the 

 section at Errol, when fully exposed, and a good typical section in the neighbourhood of 

 Paisley. 



The Errol clay contains certain species, either absent or extremely scarce in the 

 west, mixed with others entirely identical ; and the high northern character of these 

 peculiar species may either indicate a slight precedence in time on the part of the eastern 

 beds and the consequent prevalence of a sterner climate ; or it may (as is far more pro- 

 bable) be explained by local peculiarities of depth and exposure. The remarkable conformity 

 between the two series of clays in their sequences and characteristics, unmistakeably 

 proves, however, that they must have been formed by the exercise of the same physical 

 forces acting under the same conditions and with similar degrees of intensity and power. 



The whole of the Mollusca found, specially in the eastern fossil beds, range within the 

 Arctic circle and off the east coast of North America, and do not live off the coast of Great 

 Britain ; and although the percentage of species collected in the several beds must always 

 be shifting with fresh discoveries, there seems no doubt that in such glacial clays as those 

 of Errol on the east of Scotland, as compared with those upon the west, there is a pre- 

 ponderance of Arctic and North-east-American forms. 



The following species of an extremely Arctic character have not yet been found in 

 the west: — Crenella faba, Leda limatula, Mesalina erosa, Mesalia reticulata, Thracia 

 myopsis. 



Leda ardica is a characteristic fossil at Errol ; every lump of clay in some parts of the 

 pit is crowded with them. On the west it is extremely scarce. One specimen, evidently 

 fossil, was dredged by Mr. Jeffreys ; another was obtained from a sand bed (glacial) at 

 Stevenson ; and a few specimens have been found in Cleshmahew's Tile-works and 

 Terally Brick-works, near Stranraer. 



Pecten Grcenlandicus is also far from uncommon at Errol ; as yet only one young 

 specimen has been found in the west; and that in the bed beneath the Boulder Clay at 

 Tangy Glen. 



Modiolaria discors, var. IcBviyafa, is also abundant at Errol, while only rare specimens 

 of the fry have been found in the west. 



