174 H. HICKS ON THE SUCCESSION OF 



until last summer ; nor were the fossils previously discovered suffi- 

 ciently important to mark it as a distinct subgroup. Last August 

 I was fortunate enough to find, again in the Llanvirn quarry, 

 which is near the centre of the series, a most interesting and im- 

 portant group of fossils, distinct from any previously discovered in 

 any part of the Arenig group at St. David's. Most of the species 

 are new ; and amongst the Trilobites is one genus (Placojiaria) 

 previously unknown in Britain, and found only in Prance, Spain, 

 and Bohemia. The fauna is, on the whole, very rich in Trilobites ; 

 and several genera appear here for the first time in the succession, 

 as Illcenus, Illamojpsis, Barrandea, Phacoj)S, and Placoparia ; but 

 along with them also we find the genera Gdlymene, Trinucleus, 

 and JEglina, which had already appeared in the earlier series. The 

 fauna also contains many species of Graptolites; chiefly of the genera 

 Diplograptus and Didymograjytus ; and there are also unusually 

 large Cephalopods, Gasteropods, Brachiopods, and Lamellibranchs. 

 The list, which is a very complete one for so early a series, will be 

 found in the Table accompanying this paper ; and I have added 

 descriptions of all the new forms, except the Graptolites, which will 

 be described by Messrs. Hopkinson and Lapworth in their paper. 

 The fauna is very like that found in the Angers slate in Prance, and 

 in M. Barrande's Etage D. 1, in Bohemia, the almost total absence 

 of which in England hitherto has been frequently noticed; and it 

 now enables us readily to recognize the position of those beds in 

 relation to the general succession in this country. 



At the south end of Ramsey Island some beds of the Upper 

 Arenig are seen in the form of black shales, between masses 

 of felspathic quartz-porphyry, and faulted against beds of the 

 Harlech group. In these shales at Porth-hayog, in the summer of 

 1878, Prof. Ramsay, Mr. Etheridge, Mr. Homfray, and myself 

 found several species of Graptolites, also Calymene jparvifrons, a 

 new Trinucleus, and some Brachiopods. These fossils indicate a 

 position for the beds intermediate between the Llanvirn quarry and 

 the Lower Llandeilo. 



In the foregoing description of the series which collectively form 

 the Arenig group, I have shown that the sequence of the rocks at 

 St. David's is very perfect, and that there is no evidence of any 

 great physical change from the beginning to the close of the epoch. 

 The lower and upper portions of the series are very homogeneous 

 sediments ; but the middle is rather more varied in character, and 

 yet not sufficiently so to indicate any great or sudden change at the 

 time it was deposited. There are three distinct faunas in the group ; 

 and only a few species pass from the one to the other. They are 

 closely allied, however, by their general facies ; and the group is 

 undoubtedly, both palseontologically and lithologically, one of the most 

 characteristic and important in the early deposits. The succession 

 observed at St. David's is more perfect, and the thickness of the 

 series greater than is found in any other British area ; but the 

 group occurs, with nearly the same general succession, also in 

 Carnarvonshire and Shropshire. 



