G. L. EUm — GraptoUte Zones in Arenkj Rocks. 209 



Possibly the shales, which seem to be responsible for the formation 

 of the little bay south of Trwyn Gaelic, and thence run south-east, 

 belong also to this zone of D. bifidus, but I have, been unable to 

 determine this with certainty. 



The o'l'eater part of the black slaty beds forming the cliffs at 

 Road Ucliaf and Road Isaf have the fauna characteristic of the 

 higher part of the zone of I), extensus ; the beds dip landwards at 

 a very high angle, and run approximately north and south. The 

 following fossils occur : — 



Bklijm.ograpUi.s extensus, Hall. Dendroyraptus arbuscula, Salt. 



D. sparsus, Hopk. D.Jlexuosus, Hall. 



D. pennatulus, Hall. J), divergens, Hall. 



D. gibberulus, Nich. I), dijfusus, Hall. 



Trigonograptus ensiformis, Hall. J). Momfrayi, Hopk. 



T. truncatus, Lapw. Ptilograpius Hicksi, Hopk. 



Callograptus radiaius (Salt.). P. cristata, Hopk. 



The higher beds seen possibly represent the lower beds of the 

 zone of B, hirundo, since they are crowded with a species of graptolite, 

 I), sparsus, closely allied to J), hirundo, if not merely a variant of it, 

 and the typical fossil, so far as I am aware, has not yet been recorded 

 from the island ; the higher beds of the zone of I), hirundo may be 

 present west of Aber Mawr, but they are not well exposed owing 

 to the nature of the country. The rocks may be faulted as Hicks' 

 map would indicate, yet it is remarkable that the graptolite zones 

 occur precisely where they might be expected were the succession 

 normal. 



(c) Whitesand Bay. 



Slaty beds containing an abundant Dendrograptus fauna ^ are found 

 on the mainland in Whitesand Bay, north of Trwyn Hwrddyn ; they 

 were considered by Hicks to be of the same age as the beds of 

 Road Uchaf and Road Isaf, but the evidence tends, I think, to 

 show that these Whitesand Bay beds are considerably lower in the 

 Arenig Series than any seen on Ramsey Island. 



These slaty beds are highly inclined, and dip a few degrees 

 west of north at a high angle, they pass upward into a series of 

 well-bedded slates with gritty bands seen in the old quarry in Forth 

 Llenog, and from these beds there have been obtained — 



Azygograptus Hicksi, Hopk. Bendrograptus arbuscula, Hall. 



Tetragraptus Amii, Lapw. B.flexuosus, Hall. 



T. quadribrachiatus, Hall. Callograptus elegans, Hall. 



Bidymograptus {extensus type). 0. Salteri, Hall.^ 

 Clematoyraptus impltcatus, Hopk. 



This fauna suggests the lower part of the zone of D. extensus, and 

 hence these beds are rather lower than any seen on Ramsey Island ; 

 thus the beds which underlie them to the south must be still lower 

 in the series. 



These slates pass up into others containing a rich fauna, Trilobites 

 and Brachiopods, but no identifiable graptolites. These are also 

 seen at Forth Melgan, east of St. Davids Head. 



' Cf. lists of Hicks and Hopkinson, loc. cit. 



DECADE Y. — YOL. I. — NO. Y. H 



