148 TRILOBITES OF GIRVAN. 



reduced) and the neck. In other respects these head-shields do not offer any 

 marked or constant points of difference from the Drummuck examples, and the 

 pygidia are indistinguishable. The shape and relations of the glabella resemble 

 Sphaerocoryphe Jmbneri, Schmidt, 1 but this species has no lateral spines on the 

 margin of the cheeks. S. granulatus, Angelin,- which also bears much resem- 

 blance, has only one lateral spine instead of two. The Balclatchie specimens are 

 generally of smaller size than the others, but whether they are a distinct variety 

 or species is doubtful, owing to the absence of satisfactory material. 



The type form comes from Thraive Glen and the Starfish Bed, and is closely 

 allied to Sphaerocoryphe dentata, Ang., S. granulata, Aug., and S. cranium, Nieszk. 

 The characters of the head-shield of Sphaerocoryphe separate it from the genus 

 Staurocephalus, sens, str., and those of its pleurae from Nieszhowslcia. The extra- 

 ordinary development of the first and only pair of pygidial pleurae recalls the 

 condition of Deiphon forbesi, in which the lateral lobes of the pygidium have 

 likewise disappeared. 



Collections. — Mrs. Gray (f . M.) ; Museum of Practical Geology ; Edinburgh 

 Museum; Woodwardian [Sedgwick] Museum. 



Horizons and Localities. — Type form. Drummuck Group (U. Bala) : Thraive 

 Glen; Drummuck; (and Starfish Bed). Whitehouse Group (M.Bala): Shalloch 

 Mill. Variety. Balclatchie Group (Llandeilo) : Balclatchie ; Dow Hill. 



6. Cheirurus (Youngia) trispinosus, Young, 18G8. Plate XIX, figs. 8 — 12. 



1868. Cheirurus trispinosus, Young, Proc. Nat. Hist. Soc. Glasgow, vol. i, p. 169, pi. i, figs. 4, Gb. 

 1876. Cheirurus trispinosus, Armstrong and Young, Cat. West. Scot. Foss., p. 15. 

 1878. Cheirurus trispinosus, Etheridge, jun., Proc. Roy. Phys. Soc. Edinb., vol. iv, p. 173. 

 — Cheirurus trispinosus, Nicholson and Etheridge, Mon. Silur. Foss. Girvan., fasc. i, p. 105, 

 pi. vii, figs. 10—17. 

 1885. Youngia trispinosa, Lindstrom, Forteckn. Gotl. Silur. Crust. (Ofv. K. Vet. Akad. Forhandl., 

 no. 6), p. 49. 



1898. Youngia trispinosa, Reed, Geol. Mag. [4], vol. v, p. 212. 



1899. Cheirurus trispinosus, Mem. Geol. Surv., Silur. Rocks Brit., vol. i, Scotland, pp. 672, 688. 



Specific Characters. — Head-shield probably semicircular in shape, with long 

 spines to genal angles. Glabella subcircular, strongly convex, truncated posteriorly 

 by occipital furrow, slightly overhanging in front, nearly three fifths the total 

 width of the head-shield (without the spines), and as broad as long. Three pairs 

 of lateral furrows present; anterior pair short, shallow, inconspicuous, and far 

 forward ; middle pair well marked, curved back, oblique, extending over about 

 one third the width of the glabella on each side; posterior or basal pair deeper 



' Schmidt, 'Rev. Ostbalt. Silur. Trilob.,' pt. i, p. 168, pi. viii, figs. 11, 12. 



- Angclin, ' Pal. Scand.,' p. 76, pi. xxxix, fig. 4; and Schmidt, op. cit., j>. 169, pi. viii, figs. 17 — 19. 



