14i; TRILOBITES OF GIRVAN. 



where I have examined them, agree with that figured by Salter erroneously (op. 

 cit., pi. vii, fig. 22) as the head-shield of St. unicus. It is not certain whether 

 Wyville Thomson considered the head-shields or the pygidium as the type of his 

 MS. species, St. maclareui. As he apparently confused two distinct species under 

 this name by associating a detached pygidium of one (Gh. unicus) with the detached 

 head-shield of another (Sphxrocoryphe thomsoni, sp. nov.), it seems better to let the 

 MS. name St. maclareui drop. Fortunately, his type of Gh. unicus is clear and 

 sufficient for identification and comparison. 



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

 [Sedgwick] Museum ; Hunterian Museum. 



Horizons and Localities. — Balclatchie Group (Llandeilo) : Balclatchie ; Pen- 

 whapple Glen. ? Balclatchie Conglomerate. 



5. Cheirurus (Sphserocoryphe) thomsoni, sp. nov. Plate XVIII, fig. 17; Plate XIX, 



figs. 1-7. 



1865. Staurocephalus ? unicus, Salter (e. p.), Mori. Brit. Trilob., p. 86, pi. vii, fig. 22 (non figs. 23, 24). 



r 1876. Deiphon forbesii, Armstrong and Young, Cat. West Scot. Foss., p. 16. 



? — Staurocephalus unicus, Armstrong and Young, ibid., p. 16. 



1877. Staurocephalus ? unicus, Woodward (e. p.), Cat. Brit. Foss. Crust., p. 59. 



1878. Staurocephalus maclareni, Etheridge (e. p.), Cat. Camb. Silur. Foss. Mus. Pract. Geol., p. 39. 

 — Staurocephalus ? unicus, Nicholson and Etheridge (e. p.), Mon. Silur. Foss. Grirvan., fasc. i, 



p. 118, pi. viii, figs. 9, 12—16 (non figs. 10, 11) ; and p. 120, woodcuts 6 a, b. 

 1899. Staurocephalus unicus (e. p.), Mem. Geol. Surv., Silur. Rocks Brit., vol. i, Scotland, pp. 509, 

 674, 689. 



Specific Characters. — Head-shield transverse, broadly semicircular, with 

 prominent glabella, consisting of large, swollen, subglobular anterior portion over- 

 hanging front margin, and about one third the width of the head-shield, sup- 

 ported by short, narrow neck, much depressed, with one pair of small lateral 

 nodular lobes. Surface of glabella finely tuberculate. Axial furrows deep. Fixed 

 cheeks triangular, convex, but much lower than globular portion of glabella; 

 surface pitted. Narrow, smooth, lateral border to fixed cheek, with lateral edge 

 furnished with two short obtuse spines. Occipital segment narrow, smooth. 

 Genal angles furnished with strong curved spines, reaching back to about third 

 thoracic pleura?. Facial sutures acutely bent, the branches meeting at the eye at 

 an angle of about GO . Eyes small, prominent, placed far forward, but behind 

 middle of globular portion of glabella. Free cheeks very small, triangular, placed 

 on the front margin of the head-shield. 



Thorax of ten segments ; axis cylindrical, convex, about half as wide as pleural 

 portions. Pleurae gently curved downwards and backwards; inner portion 

 straight, semicylindrical, feebly convex, but suddenly raised at fulcrum into a 



