On the Dentition o/'Pleuroplax (Pleurodus). 291 



sutures, in which character it approaches Nautilus (Herco- 

 glossa) franconicus, Oppel. 



Horizon. Calcareous Grit and Kelloway Hock. 



Localities. Wiltshire ; Marcham, Berkshire (Calcareous 

 Grit). Scarborough, Yorkshire (Kelloway Rock). 



XL. — On the Dentition of Pleuroplax (Pleurodus), A. S. 

 Woodw. By James W. Davis, F.L.S. 



[Plate XIII.] 



In May 1879 * I described the teeth and spines of Pleuroplax 

 (Pleurodus) affinis, Agass., occurring in a thin shale above 

 the Better-bed Coal of Clitton and Lowmoor, near Halifax. 

 A comparison of these spines with similar ones from the 

 Staffordshire Coal-field, in the cabinet of Mr. John Ward, of 

 Longton, showed them to be closely related. In connexion 

 with one of the Staffordshire spines were a few fragments of 

 teeth, referred with probability to the genus Helodus, and the 

 inference was drawn that the two genera had similar spines. 



Mr. Ward has just issued an admirable account of the North 

 Staffordshire Coal-field f, in which he refers to the occur- 

 rence of numerous teeth of Helodus simplex, Ag., in associa- 

 tion with a spine much resembling that of Pleuroplax, the 

 full description of which he reserves to a future time. Mr. 

 Ward also describes a specimen in his collection from the 

 Northumberland Low Main Coal of the jaw of Pleuroplax 

 Rankinei, Ag. It " is somewhat in the shape of a horseshoe, 

 with a blunt rounded extremity, the articular ends expanded. 

 Both rami support teeth, several of which unfortunately are 

 displaced. Those in position are arranged upon the jaw with 

 the lateral expansions pointing antero-posteriorly. The ante- 

 rior teeth are relatively narrower than the posterior. The 

 most posterior tooth, at least, has the summit of the crown 

 crenulated." 



Recently, whilst on a visit to Glasgow with my friend Mr. 

 A. Smith Woodward, we found two specimens of Pleuroplax 

 which prove not only that the two genera had similar spines, 

 but that they are one species with the same spine. One of 

 the specimens is from the University Museum, Glasgow, and 



* Quart. Journ. Geol. Soc. vol. xxxv. p. 181, pi. x. figs. 1-11. 



t " The Geological Features of the North Staffordshire Coal-fields/' 

 "by John Ward : Trans. N. Stafford. Inst, of Miniug and Mechanical 

 Engineers, vol. x. (1890). 



