50 DEVONIAN FAUNA. 



are alike, and the great difference in apical height may be seen by comparing the 

 two first figures on PI. VI. No doubt this is partly due to pressure during 

 fossilization, but the amount of natural variation was evidently very great. 

 Nevertheless, there do not appear to be any definite characters by which any of 

 them might be specifically distinguished from the rest, and there seems every reason 

 to suppose that they all belong to a single very variable species. 



This view is borne out by a comparison with the foreign forms described by 

 J^. A. Romer, with which our English specimens evidently agree. They are shown 

 by Kayser to form part of an extremely variable species, to which he gives the 

 name G. hercynicus, but which, as it appears to me, the laws of nomenclature oblige 

 us to call either G. Bischofii or G. acutus. The limits which Kayser assigns to his 

 species are still wider than those shown by our Pilton specimens. There is no 

 English evidence of the elongate G. acutissimus or the smooth G. selcanus which he 

 includes among its varieties. Even, however, excluding the two latter forms, the 

 variation of the German shell is fally as great as ours. 



Barrois, it is true, re-divides Kayser's species, but he intimates that he does so 

 simply on artificial grounds, and it seems possible that some of his other forms, 

 besides those enumerated above, may also belong to it. 



Tschernyschew's P. cuUellus is a flattened form almost exactly like the 

 specimen here figured on PI. V, fig. 20, and cannot possibly be specifically dis- 

 tinct from that shell. 



G. conicus, Hall, seems exactly to correspond. 



The American Carboniferous form Platyceras quincyense, McChesney, appears 

 to have its base more oblique, so that the front margin seems perpendicular to it. 

 This, if not a mere accident, is possibly not more than a varietal diff'erence. It 

 seems to fall well within the limits of the present species, but its apex is unknown. 



One of Mr. Porter's Pilton specimens is interesting from the fact that it is 

 attached to, and seems totally to envelope, a specimen of Actinocrinus Porteri. 



Affinities. — Platyceras Lorieri, de Verneuil/ is distinguished by its whorl being 

 much more incurved and considerably overhanging the inner margin of the mouth. 



Platyceras dentalium, Hall,^ differs by having the longitudinal ribs and furrows 

 twisted instead of straight. 



Gapuhis quadratus, Maurer,'^ seems distinguished by its quadrate section, its 

 sharp apex, and its perpendicular inner side. 



1 1850, de Verneuil, ' Bull. Soc. Geol. Pr.,' ser. 2, vol. vii,p. 779 ; and 1881, (Ehlert, 'Mem.- Soc. 

 Geol. Fr.,' ser. 3, vol. ii, p. 14, pi. ii, figs. 1 a — d. 



2 1861, Hull, • Desc. New Species,' p. 1 ; and 1879, Hall, ' Pal. N. Y.,' vol. v, pt. 2, p. 2, pi. i, 

 figs. 3 — 8 ; and 1881, (Ehlert, ' Mein. Soc. Geol. Fr.,' ser. 3, vol. ii, p. 15, pi. ii, figs, 2 a — c. 



^ 1885, Maurer, ' Abhandl. Grossb. Hessisch. Geol. Landes.,' vol. i, pt. 2, p. 243, pi. x, figs. 

 26—28. 



