162 SILURIAN TRILOBITES. 



AsAPHUS PLATTCEPHALUS, Bronn. Lethsea, vol. i, p. 115, pi. ix, fig. 8, 1835. 



— GIGAS, Emmrich. Dissert. 32, 1839. 



IsoTELUS GIGAS, MUne-Edwards. Crust, vol. iii, p. 298, 1840. 

 AsAPHUS PLATYCEPHALUS, Buckland. Bridgw. Treat., vol. 2, p. 75, 1840. 



— — Burmeister. Org. Tril., p. 127, pi. ii, fig. 12, 1843; and Ray 



edition, p. 110, 1846. 

 IsoTELUS GIGAS ; I. PLANUS, PortlocJc. Geol. Report, p. 295, pi. viii, fig. 7 ; pi. vii, 



figs. 2, 3 (except pi. viii, figs. 2, 3,). I. ovatus ; I. 

 scLEBOPS. Id., Pi. viii, fig. 5 ; pi. x, fig. 2. I. Pow- 

 isH (not of ' Sil. Syst.') Id., p. 297, pi. vi, fig. 1, 1843. 



— MEGISTOS, Locke. Americ. Journ. Science, vol. xiii, p. 366, 1842; Trans. 



Assoc. Amer. Nat. and Geol., vol. i, pi. vi, 1843. 



— GIGAS, Hall. Pal. New York, vol. i, p. 231, pi. Ix, fig. 7 ; pi. Ixi, figs. 3, 4 ; 



pi. Ixii, figs. 1, 2 ; pl. Ixiii, 1847. 



— — Billings. Geol. Canada, p. 184, fig. 182. A. platycephalus, ib., 



fig. 183? 1863. 

 AsAPHUs (IsoTELUs) GIGAS, Salter. Mem. Geol. Surv., Decade xi, sec. 3, p. 1, pl. iii, 1864. 



A. {Isot.) ovato-ohlongus, lavis,lateribus rectis ; capite pygidioq^ue ejusdem magnitudinis, 

 subtrigonis et hgperbolicis. Capitis sulci axales mhiime profundi. Oculi modici et pone 

 medium caput. Sutura facialis intra marginem, cui parallela est ; labrum ad basim angustmn, 

 in cornua longa parallela productum. Thorax axe pleurisque ejusdem latitudinis, fulcro 

 ad tertias posito. Cauda axis indistinctus conicus longusque ; in latere quoque sulcus 

 superior tantum videtur, cateri desunt ; oculi modici pone medium caput. 



One sometimes persuades oneself of a thing by continually asserting it, and as this 

 species has been continually quoted of late years as the /. gigas, it has come to be gene- 

 rally accepted as such. There are some differences from American specimens, however, 

 which may possibly cause its separation when we have more abundant material. I have 

 used General Portlock's original specimens, and, believing the species identical with the 

 common American form, follow most writers in adopting De Kay's name. It must have 

 been published at nearly the same time as Mr. Stokes' \\Qm.e platgcephalus, though the 

 paper of the latter, in the Geol. Trans., was read early in 1823. Moreover the fossil 

 described by Stokes has a broader form and smaller eyes than the narrower one commonly 

 known as /. gigas. But if these differences be regarded as due to sex, the /. gigas being 

 the S , and /. platycephalus the ? form, there would be no difficulty in accepting all 

 as of one species. 



In the Decade XI of the Geol. Survey, I have fully described this fossil, and I do not 

 know that I can do better than quote that description. I look upon all Professor Green's 

 casts as one species. Professor Hall does the same ; and Portlock's diiRculties arose out 

 of an attempt to identify his variably preserved specimens with the named casts of 

 Professor Green. There is no occasion here to distinguish 1. gigas from /. Powisii ; 

 they belong to different subgenera. 



I would only repeat here that there is no warrant, in any fragments I have seen or 



