48 TRILOBITES OF GIRVAN. 



cutting the posterior margin of the head-shield much further out, by the axial 

 furrows being weaker and more curved outwards (in adult examples) behind the 

 eyes, and by the distinctly spined genal angles. Clarke T has remarked that the 

 usual basis of distinction between A. gigas, De Kay, and A. maxim us, Locke, is that 

 the former has no genal spines, while the latter possesses them. Schmidt 2 considers 

 Isot. remigium,, Eichw., to be closely allied to Salter's Isot. gigas; but there is con- 

 siderable doubt if Salter's Isot. gigas is the same as De Kay's Asaphus gigas. 



With A. scutalis, Salter, 3 this Girvan form agrees in the shape of the head-shield, 

 the axial furrows, and the proportions of the glabella, but the eye is rather further 

 back, and the facial suture bends out rather more sharply behind the eye and cuts 

 the posterior margin at a more acute angle. 



The head is not as broad as in A. rectifrons, Portl., 4 nor the eye as far back. In 

 neither A. scutalis nor A. rectifrons are the genal angles spined, but in Isot. 

 platyrhachis, Steinhardt, 5 which in cranidial respects appears closely allied to this 

 Girvan form, this feature is found. The smaller specimens of A. rnaximus, Locke, 

 bear a considerable resemblance to A. instabilis, but the head-shield seems to be 

 narrower and more pointed. 



With regard to the thorax, the axis is narrower than in I. platyrhachis, but is 

 about the same width as in A. sen tali*. The fulcrum in the latter species is, however, 

 nearer the axis than in the Girvan form, and the pleural groove less strong. More- 

 over the axis of the pygidium is narrower in A. scutalis and has a double row of 

 10 — 11 puncta, and the caudal fascia is broader. The pygidium of the Girvan 

 form bears some resemblance to A. rectifrons, Portlock, as interpreted by Salter," 

 the pygidium being attributed to Isot. arcuatus by Portlock. 7 There seems to be 

 much difference of opinion as to what is the typical shape of the pygidium in the 

 American allied species A. gigas and A. max! mns. The British form called A. gigas 

 has too parabolic a pygidium for either, and the axis shows no segmentation. 

 According to Clarke {pp. cit.) mature individuals in the above-mentioned species 

 lose the segmentation, so that its presence or absence is merely a sign of age. 

 Schmidt 8 says that the genus (or sub-genus) Isotelus occurs exclusively in the higher 

 Ordovician beds (except the doubtful Isot. ? stacyi, Schm.), but this is certainly 

 not the case in the British Isles. 



1 Clarke, "Lower Silur. Trilob. Minnesota," 'Final Rep. G-eol. Nat. Hist. Surv. Minnes.,' vol. iii, 

 1894, pt. ii, p. 701. 



2 Schmidt, 'Rev. Ostbalt. Silur. Trilob.,' Abtli. v (1901), p. 90, pi. x, fig. 17; pi. xi, figs. 1—7. 



3 Salter, ' Mon. Brit. Trilob.,' p. 169, pi. xxv, figs. 2, 3. 



4 Portlock, ' Geol. Rep. Londond.,' p. 298, pi. ix, figs. 1 a, b, and pi. viii, figs. 2, 3, 7. 



5 Steinhardt, ' Preuss. Trilob.,' p. 24, pi. i, fig. 10. Schmidt, op. cit., p. 91, pi. x, figs. 13—16; 

 pi. xi, fig. 8. 



,; Salter, op. cit., p. 166, pi. xxv, fig. 8. 

 7 Portlock, op. cit., pi. ix, figs. 2, 3. 

 s Schmidt, op. cit., p. 87. 



