part 4] upper ordoyiciajN" of the berwtn hills. 



493 



farm (a 4, fig. 2), it appears that the shale with poorly-preserved 

 graptolites is thinner ; but here are bands of black muddy lime- 

 stone. These limestones contain a few fossils, mainly hinged 

 brachiopods similar to those of the shales, though as a rule not quite 

 so dwarfed. It was in these beds that the new tunnel for the 

 water-main from Lake Vyrnwy was begun ; and a natural exposure 

 may be studied a few yards south-west of the tunnel-mouth, where 

 the limestones form a cliff on the western bank of the stream 

 (north-west of «1 in fig. 2). 



It is believed that this horizon represents the greater part, or 

 more probably the whole, of the Gwern-y-brain Beds of the 



Fig. 2. — Sketch-map 1 : Aber MarcTinant. 



[The Phillipsinella mudstone (Ashgillian) is represented by open dots ; the 

 Black Shale Group (Caradocian) by a striped band, and the sandy mud- 

 stones (Caradocian) by black dots.] 



Welshpool area described by Dr. A. Wade. 1 The fauna in the 

 Gwern-y-brain Beds is exactly identical with, but more varied 

 than, that of the Pen-y-garnedd Shales, and, if we take the two in 

 conjunction, the following points will be noticed : — (a) the dwarfed 

 size of all forms ; (b) the abundance of individuals of a few 

 species; ( c) the thin-shelled character of the fauna; and (d) the 

 abundance of small bellerophons, gastropods, ostracods, and horny 

 brachiopods, together with dwarfed graptolites. 



The characters of this fauna clearly indicate conditions un- 

 favourable for the growth of the thick-shelled forms usually found 



1 Q. J. G. S. vol. lxvii (1911) pp. 428-31. 



