18 



TRILOBITES OF GIRVAN. 



Dimensions. — 



Length of middle shield to point of ogive . . 27'0 mm 



Width of „ at base . . . 26-0 



Width of glabella at base . . . 16"0 



Width of „ between eyes . . . 11*0 



Distance of eyes from base . . . lO'O 



Maximum width of middle shield between facial sutures 



in front of eyes .... 18"0 



Distance of tubercles on glabella from base . . 7-0 



Length of eye-lobes . . . (about) 4-0 



Length of pygidium .... 25"5 



Width of „ at front end . . . 22-0 



Width of axis at front end . . . 9*0 



Bemarhs. — No example of the head- shield with free cheeks attached or of 



detached free cheeks referable to this species have come under my notice from 



Balclatchie, but if it is correct to refer the Ardmillan specimen previously figured 



by me as A. gigas to this new species, the free cheeks may have their characters 



described as follows : Triangular, elongated, narrow, with genal angle produced 



back into short acute point. The resemblance of the head-shield of this species 



to the form (or forms) usually referred to A. gigas (De Kay) by Salter^ and 



others is very close ; the pair of small tubercles on the glabella is, how^ever, a 



distinctive feature. The thoracic rings show no peculiar characters. But the 



pygidium is narrower, more elongated, and more pointed than in A. gigas, 



and the axis is obsolete on the surface. In these respects it specially resembles 



A. (Isotelus) platyrJiachis, Steinh.^; but the sharply pointed extremity more 



recalls such species of Megalaspis as M. acutlcauda, Ang.,^ though there can be no 



doubt that our species is referable to the subgenus Isotelus. The classification of 



Asaphidge has been recently revised by Raymond,* with especial reference to Isotelus, 



but only a preliminary sketch of his conclusions has at present been published. 



Horizon and Locality. — Balclatchie Group (Llandeilo) : Balclatchie ; Ardmillan. 



Genus STYGINA, Salter. 

 Stygina latifrons, Portlock. Plate III, fig. 7. 



1904. Stygina latifrons, Portlock, Reed, op. cit., pt. ii, p. 50, pi. vii, fig. 10. 



Only the pygidium of this species has previously been recognised in the Girvan 



1 Salter, ' Mon. Brit. Trilob.,' p. 161, pi. xxiv, figs. 1—5. 



3 Schmidt, 'Rev. Ostbalt. Silur. Trilob.,' pt. v, lief, ii (1901), p. 91, pi. x, figs. 13—16. 

 3 Schmidt, op. cit., pt. v, lief, iv (1906), p. 42, pi. v, figs. 1 — 8; pi. vi, figs. 1 — 6. 

 * Raymond, ' Trans. Roy. Soc. Canada' [3], vol. v, sect, iv (1912), pp. Ill — 120. 



