CHBIRURUS UNICUS. 143 



1865. Staurocejjhalus ? maclareni, Salter, ibid., p. 86. 



— Staurocejjhalus ? unicus, Salter (e. jk), Mem. Geol. Surv., dec. xi, art. 5. 



? 1876. Staurocejjhalus ? unicus, Armstrong and Young, Cat. West. Scot. Foss., p. 16. 



1877. Staurocejjhalus '1 unicus, Woodward (e. p.), Cat. Brit. Foss. Crust., p. 59. 



1878. Staurocejjhalus? unicus, Nicholson and Etheridge (e. jj.), Mon. Silur. Foss. Girvan, fasc. i, 

 p. 118, pi. viii, figs. 10, 11 (non figs. 9, 12 — 16). [Fig. 11 is the type specimen of 8tau.ro.? 

 maclareni, Wyv. Thorn. MS., Coll. Mus. Pract. Geol.] 



1878. Cheirurus gelasinosus ?, Nicholson and Etheridge, ibid., p. 100, pi. vii, figs. 5, 6. 



— Staurocejjhalus maclareni, Etheridge (e. j).), Cat. Camb. Silur. Foss. Mus. Piact. Geol., p. 89. 

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



674, 689. 



Specific Characters. — Head-shield broadly semicircular, nearly three times as 

 broad as long. Glabella broadly oval, truncate at base, strongly convex, rising 

 twice as high as the cheeks ; three pairs of lateral furrows, the basal pair strongly 

 oblique and incompletely marking off triangular basal lobes, the two anterior pairs 

 of furrows less oblique, but parallel with one another, the more anterior pair close 

 to the front end of the glabella, and all the furrows extending about one third of 

 the way (or rather less) across the glabella. The basal lobes extend farther along 

 the side of the glabella than the middle or first lateral lobes, and are larger than 

 either. Surface of glabella ornamented with small, irregular, somewhat sparse 

 tubercles. Fixed cheeks gently convex, pitted, more than twice as wide as glabella. 

 Narrow anterior wing present to fixed cheek in front of eye, joining on to smooth, 

 flattened, pre-glabellar border. Eye of moderate size, situated opposite middle 

 lateral lobe or a little behind it. Occipital ring at base of glabella of moderate 

 size, of regular width, marked off by strong furrow. Behind the cheeks the neck- 

 segment widens towards the genal angle, which is produced into a short point. 

 The neck- segment is tuberculated, like the glabella. 



Thorax of twelve segments. Axis, strongly convex, narrow, half as wide as 

 pleural portions, cylindrical, very slightly tapering posteriorly. Axial rings simple. 

 Pleurae horizontally extended, straight as far out as the weak fulcrum which is 

 situated at about two thirds their length; beyond the fulcrum they are gently 

 curved backwards and a little downwards. Each pleura is narrow and of uniform 

 width, except beyond the fulcrum, where it tapers gradually to a pointed extremity ; 

 a median longitudinal furrow runs along it, marked with a row of minute pits. A 

 single longitudinal row of small tubercles ornaments the anterior and posterior 

 portions of each pleura. 



Pygidium short, broad, with much developed first pair of pleuras. Axis con- 

 sists of two complete rings, followed by a triangular piece forming more than 

 half the axis, and bearing an incomplete transverse groove near its anterior end. 

 Axial furrows indistinct, the second axial ring merging into the base of the 

 second pair of pleurae. Lateral lobes consisting of two pairs of pleurae, of which 

 the first is much the larger, being produced into long, backwardly directed, and 



