TRIARTHRUS BBOKI. 20 



18(31. Triarthrus beckii, Barrande, Bull. Soc. G-eol. France, ser. ii, p. 269, pi. v, figs. 11, 12. 



1863. Triarthrus beckii, Logan, Geol. Canada, p. 202, fig. 200. 



1869. Triarthrus becki, Linnarsson, Vestergotl. Canib. Silur. Arlagr., p. 70, pi. i, fig. 27. 



1876. Triarthrus beckii, Walcott, Trans. Albany Instit., vol. x, p. 23, pi. ii, figs. 1 —16. 



1878. Cheirurus, sp., Nicholson and Etheridge, Mon. Silur. Foss. Grirvan, Ease, i, p. 107, pi. vii, fig. 19. 



1893. Triarthrus becki, Matthew, Amer. Journ. Sci., ser. iii, vol. xlvi, p. 121, pi. i. 

 Triarthrus becki, Beecher, ibid., pp. 361, 467. 



1894. Triarthrus becki, Beecher, Amer. Geol., vol. xiii, p. 38, pi. iii. 



Triarthrus becki, Beecher, Ainer. Journ. Sci., ser. iii, vol. xlvii, p. 298, pi. vii. 



1895. Triarthrus becki, Beecher, Amer. Geol., vol. xv, p. 91, pis. iv, v. 



Triarthrus becki, Beecher, ibid., vol. xvi, p. 166, pi. viii, figs. 12 — 18; pi. x, fig. 1. 



1893. Triarthrus beckii, Matthew, Trans. N. Y. Acad. Sci., vol. xii, p. 237, pi. viii. 



1894. Triarthrus beckii, Walcott, Proc. Biol. Soc. Washington, vol. ix, p. 89, pi. i. 

 Triarthrus beckii, Walcott, Geol. Mag., dec. iv, vol. i, p. 246, pi. viii. 



Specific Characters. — Glabella subquadrate, nearly as broad as long; sides 

 parallel ; marked with two pairs of almost horizontal lateral furrows, very slightly 

 curved, and extending more than a third across the glabella on each side. A pair of 

 small pits in front of the anterior pair of furrows represents the first pair of lateral 

 furrows. The anterior (true second) pair of furrows is far forward, being less than 

 half the length of the glabella from its front end. Frontal lobe of glabella short. 

 Occipital furrow strong, slightly arched forward in middle, and about the same 

 distance from the posterior pair of lateral furrows as the latter is from the anterior 

 pair. Occipital segment nearly as wide as posterior lateral lobe of glabella, and 

 with median tubercle. Narrow border in front of glabella, projecting slightly in 

 middle. Axial furrows straight, strong, parallel. Fixed cheeks convex, bent down, 

 measuring at base a little over one third the width of glabella, but narrowing 

 anteriorly ; eye-lobe of moderate size, very far forward, being in front of anterior 

 pair of lateral furrows, and marked off from cheek with strong furrow. Facial 

 suture runs back in slight outward curve to meet posterior margin at about 75°. 



Dimensions. — ■ 



Length of head-shield .... 4 - mm. 

 ,, of glabella .... 2*5 „ 



Remarks. — The minute specimen figured by Nicholson and Etheridge as doubtfully 

 belonging to the genus Cheirurus (M., fasc. i, 1878, pi. vii, f. Ii') must clearly be 

 assigned to the genus Triarthrus. Several additional specimens from the same 

 locality, Balclatchie, enable me to give the above fairly complete description of the 

 head-shield. 



Affinities. — This form appears to be identical with the Swedish species attributed 

 by Linnarsson (1869) to T. becki, Green, which occurs typically in the Utica 

 Slates of America, The Swedish examples appear, however, to differ slightly 

 from the American type in having the glabella rather more quadrate and 

 the lateral furrows and eyes rather more forward; and the Grirvan specimens 



