74 Dr. Arthur Smith Woodward — Jaw of Plectrodus. 



it is thought likely to be continuous with the Yorkshire Coalfield, hy 

 others with the Kent Coalfield, but attempts to follow it in anortherly 

 direction have so far been defeated by the great thickness of Tertiary 

 sands near Antwerp. Apart from the hi»hly speculative question of 

 its extension in either of the directions named, it seems to be 

 comparable to the Kent Coalfield in its relation to the great belt of 

 folding along which the other Belgian coalfields are situaled. 



V. — Note on Plectrodus, the Jaw of an Upper Silurian Fish. 

 By Arthur Smith Woodward, LL.D., F.R.S. 



WHEN fish-remains were first discovered in the Ludlow bone-bed 

 and other horizons of the Upper Silurian series, some of the 

 fragments were regarded as toothed jaws by Agassiz, who described 

 them under the names of Plectrodus mirabilis, P. pleiopristis, and 

 Sclerodus pustul/Jerus. 1 The same fossils were afterwards considered 

 to be of Crustacean origin by M'Coy, 2 but definitely proved to be 

 fish-remains by the microscopical examination of Harley, who pointed 

 out that while they could not be teeth or jaws, they appeared to him 

 to be " the posterior spines of the cephalic plate of some Cephalaspidian 

 fish ". 3 The latter view was adopted by Lankester, who treated the 

 specimens as parts of the cornua of a small Cephalaspidian head- 

 shield which he named Eukeraspis pustuliferus.* 



Egerton 5 also described similar fossils as jaws, and among these 

 was one specimen from the Downton Sandstone ( Brit. Mus. No. 45969), 

 which I studied some years later and considered to resemble a fish- 

 jaw rather than a cornu of Eukeraspts. 6 In 1893 llohon 7 made 

 a microscopical examination of several fragments both from the 

 Ludlow bone-bed and from the well-known Upper Silurian limestone 

 in the Isle of Oesel (Baltic Sea), and concluded that while some of 

 the specimens ascribed to Plectrodus and Sclerodus were Cephalaspidian, 

 others were certainly not of this nature. He described the latter as 

 exhibiting " denticles and tubercles of vasodentine, and the base not 

 formed of true bony tissue but of a bone-like substance". In 1898 

 I found in the Museum of Neuchatel three of the original specimens 

 from the Ludlow bone-bed which were described by Agassiz (labelled 

 •'Rev. Win. Evans, 1836"), and two of these (figured in Siluria, 

 pi. iv, figs. 15, 25) appeared to me distinctly jaw-like. In 1910 



1 L. Agassiz, in Murchison's Siluria, 1839, p. 606, pi. iv, figs. 14-32, 60-2. 



2 Pterygotus pustuliferus, F. M'Coy, Quart. Journ. Geol. Soc, vol. ix, 

 p. 14, 1853. 



3 J. Harley, "On the Ludlow Bone-Bed and its Crustacean Remains": 

 Quart. Journ. Geol. Soc, vol. xvii, p. 544, 1861. 



4 E. Ray Lankester, Fishes of the Old Bed Sandstone, pt. i (Pal. Soc, 1870), 

 p. 58, pi. xiii, figs. 9-14. 



6 P. M. G. Egerton, " On some Fish-remains from the Neighbourhood of 

 Ludlow" : Quart. Journ. Geol. Soc, vol. xiii, p. 288, pi. x, figs. 2-4, 1857. 



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

 1891, p. 195. 



7 J. V. Rohon, "Die Obersilurischen Fische von Oesel" : Mem. Acad. 

 Imp. Sci. St. Petersbourg [7], vol. xli, No. 5, p. 95, 1893. 



