46 TRILOBITES OF GIRVAN. 



1843. Asaphus platycephalus, Burmeister, Organ. Trilob., p. 110, pi. ii, fig. 12. 



— Isolelus gigas, Portlock, Geol. Eep. Londond., p. 295, pi. vii, fig. 1. 



— Isotelus planus, Portlock, ibid., pi. vii, figs. 2 — 4, and pi. viii, figs. 1, 4, 7 (uon figs. 2, 3, 6). 



— Isotelus ovatus, Portlock, ibid., pi. viii, fig. 5. 



— Isotelus sclerops, Portlock, ibid., pi. x, figs. 1, 2. 



— Isotelus powlsii, Portlock, ibid., p. 297, pi. vi, fig. 1. 



1846. Isotelus gigas, M'Coy, Synops. Silur. Foss. Ireland, p. 53. 



1847. Isotelus (jigas, Hall, Pateont. New York, vol. i, p. 231, pis. lx — lxiii. 



1854. Asaphus (Isotelus) gigas, Morris, Cat. Brit. Foss., 2nd ed., p. 100. 



1855. Isotelus gigas, Emmons, Amer. Geol., vol. i, pt. 2, p. 215, pi. xvi, fig. 12. 

 1858. Isotelus gigas, Bogers, Geol. Surv. Penm, vol. ii, p. 819, fig. 610. 



1863. Isotelus gigas, Billings, Geol. Canada, p. 184, fig. 182. 

 ? — Isotelus platycephalus, Billings, ibid., fig. 183. 



1864. Asaphus (Isotelus) gigas, Salter, Mem. Geol. Snrv., dec. xi, pi. iii. 



1865. Asaphus (Isotelus) gigas, Salter, Mon. Brit. Trilob., p. 161, pi. xxiv, figs. 1 — 5; (?) pi. xxv, 

 fig. 5. 



1870. Asaphus platycephalus, Billings, Quart. Journ. Geol. Soc, vol. xxvi, p. 486, pis. xxxi, xxxii. 

 1877. Asaphus (Isotelus) gigas, Woodward, Cat. Brit. Foss. Crust., p. 25. 



1894. Asaphus (Isotelus) gigas, Clarke, Lower Silur. Trilob. Minnesota, Final Eep. Geol. Nat. Hist. 

 Surv. Minnes., vol. iii, pt. ii, p. 701. 



Remarks. — It is highly doubtful if Salter's Asaphus gigas is identical with 

 De Kay's species, but I am not in a position to decide this point. However, there 

 are a few head-shields and pygidia with fragments of the thorax from Girvan which 

 must be referred to the same form as Salter's Asaphus gigas. A well-preserved 

 head-shield from Ardmillan shows the characteristic features seen in Portlock's 

 hot. sclerops which Salter ascribes to A. gigas. This form differs from the 

 commoner Girvan Isotelus in a manner described under I. instabilis ; but the 

 pygidia are generally too poorly preserved to decide with certainty their specific 

 position. 



Collections. — Mrs. Gray ; Museum of Practical Geology. 



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



2. Asaphus (Isotelus) instabilis, sp. nov. Plate VII, figs. 2 — 8. 



1879. Asaphus gigas ?, Nicholson and Etheridge, e. p., Mon. Silur. Foss. Girvan, fasc. ii, p. 153, 



pi. x, figs. 18, 19. 

 1899. Asaphus gigas, e. p., Mem. Geol. Surv., Silur. Rocks Brit., vol. i, Scotland, pp. 509, 513, 

 514, etc. 

 ? 1899. Asaphus rectifrons, ibid., pp. 671, 674. 



Specific Characters. — Head-shield broadly parabolic, about twice as wide as 

 long, gently convex. Glabella feebly convex, faintly defined, hour-glass shaped, 

 contracted bet ween the eyes, rounded in front, smooth, about one-third the width 

 of head-shield at base. Axial furrows weak, curve inwards in their forward course 



