70 TRILOBITES OF GIRVAN. 



prepared. The Shallocli Mill specimens, in addition to being fragmentary, are 

 frequently crushed and more or less distorted ; but point for point they agree with 

 the fine Thraive Glen specimen, and I have no doubt they are identical. 



Affinities. — This species very closely resembles I. bowmani, sens, str., but it differs 

 in the strongly marked punctation of the surface, the groove round the eye continued 

 across the base of the fixed cheek, and the character of the caudal fascia. With 

 I. linnarssoni 1 the resemblance appears still closer, particularly in the characters of 

 the pygidium and the ornamentation of the surface of the shell ; but the thoracic 

 axis in the foreign form is narrower, the free cheeks broader and shorter, and the 

 caudal fascia different. The characters altogether seem sufficiently distinctive to 

 justify placing it to a new species. 



Collections. — Mrs. Gray ; Museum of Practical Geology ; Mr. Macconochie's 

 collection. 



Horizon* and Localities. — Whitehouse Group (M. Bala) : Shallocli Mill. Drum- 

 muck Group (U. Bala) : Thraive Glen. 



13. Illsenus thomsoni, Salter, 186(3. Plate X, figs. 0—8. 



1851. Illsenus, sp. ind., Salter, Quart. Jovrrn. G-eol. Soc, vol. vii, p. 171, pi. ix, fig. 3. 



1866. IUtenopsis thomsoni, Salter, Mem. G-eol. Surv., vol. iii, p. 231 (non p. 316, pi. ix, figs. 1, 2). 



— Illsenus thomsoni, Salter, ibid., p. 360. 



1867. Illsenus (Dysplanus) thomsoni, Salter, Mon. Brit. Trilob., p. 188, pi. xxviii, figs. 2—4 (? pi. 

 xxx, figs. 8—10). 



1873. Illsenus thomsoni, Salter, Cat. Camb. Silur. Foss. Woodw. Mus., p. 86. 



1876. lllsemis thomsoni, Armstrong and Young, Cat. West Scot. Foss., p. 16. 



1877. Illsenus (Dj/splanus) thomsoni, Woodward (e. p.), Cat. Brit. Foss. Crust., p. 42. 



1879. Illsenus thomsoni, Nicholson and Etheridge, Mon. Silur. Foss. Girvan, fasc. ii, p. 157, pi. xi, 

 fig. 9. 



— Illsenus nexilis ?, Nicholson and Etheridge, ibid., p. 158, pi. xi, fig. 12. 



— Illsenus murchisoni ?, Nicholson and Etheridge, ibid., p. 161, pi. xii, fig. 1. 



1882. Illsenus thomsoni, Holm, Svensk. Art. Illgenus (Bih. K. vet. Akad. Handl., vol. vii, No. 3), 



p. 49. 

 1899. Illieuus thomsoni, Mem. G-eol. Surv., Silur. Bocks Brit., vol. i, Scotland, p. 689. 



Remarks. — This species occurs in considerable abundance at "Woodland Point 

 and Penkill. The pygidia from both these localities agree with the broad form 

 figured by Salter (Mon. Brit. Trilob., pi. xxviii, figs. 2, 3), and not with the 

 narrower form with more truncate angles (figured op. cit., pi. xxx, figs. 8, 10). 

 The latter occurs at Mulloch Hill ; but apart from the pygidia there is not 

 sufficient evidence to separate the two forms. The species is also found at Rough 

 Neuk and Bargany Burn, and probably at Camregan Wood. 



1 Holm, 'Svensk. Art, Illsenus,' p. 103, pi. iv, figs. 13—27; pi. v, figs. 1—8; pi. vi, fig. 15; 

 id. ' Eev. Ostbalt. Silur. Trilob.,' pt, 3, p. 146, pi. x, figs. 10—23. 



