THE ANCIENT KOCKS OF ST. DAVID'S. 173 



first creek to the north of Trwyn-hwrddyn in Whitesand Bay ; and 

 several new forms have again been added by us to the list, which 

 now includes in all about forty-eight species of Graptolites. 



The other fossils found in this series comprise Asaphus Homfrayi, 

 Ogygia scutatrix, Trinucleus Sedgivielcii, Gonularia Homfrayi, Theca 

 sp., Lingida petalon, and Orihis remota. This subgroup is strongly 

 characterized by its rich fauna of dendroid Graptolites, the deep-sea 

 conditions prevalent at the time being doubtless favourable to 

 their growth and development, as there is no evidence of their 

 existence in any previous faunas in the succession here. 



The Middle Arenig is made up of a series of slates and flags, 

 with narrow bands of sandstones intercalated; and it has a thick- 

 ness of about 1500 feet. The beds rest conformably on the Lower 

 Arenig ; and they can only be distinguished from the latter by having 

 on the whole a more flaggy appearance. They are best exposed at 

 .Whitesand Bay, and may be said to begin at the point which 

 separates the creek north of Trwyn-hwrddyn from that in which 

 the slate- quarry occurs. They also extend into the creek called 

 Porth Melgan, directly south of St. David's Head \ and they reach 

 across that promontory to tbe north coast. In the bay on the north 

 coast, the extremity of which is marked Porth-y-dwfr, they are lost 

 for some distance ; and we do not meet them again in their proper 

 thickness until we reach the coast west of Section III., beyond a 

 place called Porth-y-rhaw, and where the Upper Arenig beds appear 

 on the coast. Along their course several masses of a rough-grained 

 greenstone may be traced running nearly in the line of bedding. 

 A few fossils were discovered in this series as far back as the year 

 1860, by Mr. Gibbs of the Geological Survey; and the well-known 

 Trinucleus Gibbsii was first discovered by him in. these beds at 

 "Whitesand Bay. Since then, however, I have been able to make 

 a very considerable addition to the fauna; and though most of 

 these fossils have already been described by the late Mr. Salter, 

 there are yet several new species to be added. The most cha- 

 racteristic fossils in this series, and the most plentiful, are the 

 Trilobites Ogygia bullina, Ogygia peltata, Trinucleus Gibbsii, JEglina 

 grandis, and Ampycc Salteri. There are also a few Graptolites ; 

 but most of these occur in the lower beds only, the middle and 

 upper beds indicating apparently conditions more favourable to 

 Trilobites &c. 



The rocks composing the Upper Arenig resemble more those of 

 the Lower Arenig than the Middle, being like them very fine- 

 grained dark slates or shales. They have a thickness of nearly 

 1500 feet, and are well exposed along the north coast, resting on 

 the Middle Arenig, and underlying the Lower Llandeilo. 



In beds of this series, at Llanrian, Prof. Hughes of Cambridge, 

 several years ago, found a species of Illcenus, which I propose now 

 to call Illcenus Hughesii ; the same species, along with a few other 

 fossils, were discovered by me also in a quarry under Llanvirn, 

 near Aber-eiddy, in 1865. 



The beds of this series, however, were not thoroughly examined 



