E. W. Hughes — Geology of Part of Carnarvonshire. 75 



Priera l described and discussed similar fossils from the Upper 

 Siliman of S. Felix, Laundos, Portugal, and decided that Plectrodus- 

 proper at least was a jaw, although Sclerodus pustuliferus was 

 undoubtedly part of the cornu of the Ceplialaspidian Eukeraspis. 



Later studies have convinced me tbat the time Plectrodus (as 

 typified by P. mirabilis) is indeed a jaw, and that it most closely 

 agrees with the toothed jaws of the Lower Devonian Acanthodian 

 Ischnacanthus. 2 I am, therefore, much indebted to Mr. W. Wickbam 

 King, F.G.S., for the opportunity of examining the microscopical 

 structure of an unusually large specimen, probably of a new species, 

 which he has recently found in the Downtonian of Baggeridge, 

 S. Staffordshire. The hard base to which the teeth are affixed proves 

 to consist of almost structureless translucent calcified tissue in which 

 there are occasional streams of elongated cellular spaces, irregular in 

 shape, and sometimes with traces of ramifying canaliculi. It thus 

 agrees exactly with the corresponding tissue in Ischnacanthus. 



In this connexion it is interesting to add that both in Oesel and in 

 Portugal separate whorls of teeth such as occur in front of the lower 

 jaw of Ischnacanthus, 3 have been found in the same rocks as Plectrodus. 1 

 On the other hand, no typical dermal tubercles of Acanthodian fishes 

 have hitherto been recognized in any Upper Silurian formation, and 

 the occurrence of Acanthodian fin-spines is uncertain. We may, 

 therefore, conclude that the toothed Acanthodians of the Lower 

 Devonian were preceded in the Silurian by fishes with similar jaws, 

 but the precise nature of these earlier fishes still remains to be 

 determined. 



VI. — Ok the Geology of the District from Cil-y-Coed to tub 

 St. Annes-Llanllyfni: IIidge (Carnarvonshire). 5 



By E. Wynne Hughes, M.Sc, F.G.S. 



(Concluded from January Number, p. 25.) 



B. Pre-Cambrian, Hhyolitic Series. — (i) At the south-west end 

 of the Craig-y-Dinas mass we find exposed a rock similar in every 

 respect macroscopically to that which is found at Cil-y-Coed. 

 Unfortunately it cannot be followed far, as the land is completely 

 under drift," the top of Craig-y-Dinas itself being capped by a thick 

 layer of drift. 



1 F. Priem, " Sur des Poissons et autres Fossiles du Silurien superieur da 

 Portugal" : Communic. Serv. Geol. Portugal, vol. viii, p. 3, pi. i, figs. 7-10, 

 1910. 



2 A. S. Woodward, Catalogue of Fossil Fishes in the British Museum, pt. ii, 

 1891, p. 20. B. Dean, "Notes on Acanthodian Sharks": Amer. Journ. 

 Anat., vol. vii, p. 209, figs. 1-10, 1907. E. S. Goodrich, in Lankester's 

 Treatise on Zoology, pt. ix, 1909, p. 190, fig. 160. 



3 A. S. Woodward, Presidential Address, Quart. Journ. Geol. Soe. , vol.lxxi, 

 p. lxvi, 1915. 



4 Campylodus sigmoides, J. V. Rohon, loc. cit., p. 52, pi. i, fig. 25, 1893 ; 

 C.(?) delgadoi, F. Priem, loc. cit., p. 5, pi. i, figs. 11-15, 1910. 



5 Plates I and II and two maps, which illustrate this paper, will be found 

 with the first part in the January Number, pp. 13 and 15. 



