LICHAS (PLATYLICHAS) LAXATUS. 103 



the front edge. The pleurae are flat, horizontal, and quite straight as far out as 

 the fulcrum, which is situated at about two thirds the width, of the axis from the 

 axial furrow. The extra-fulcral portion of each pleura (well preserved in another 

 specimen from the same locality) is gently recurved and tapers to a free point. A 

 diagonal furrow crosses the whole pleura, and is continued almost to the free 

 extremity. The fulcrum is weak and rounded. The pleura? increase slightly in 

 length towards the pygidium. The whole surface of the thorax is ornamented with 

 small tubercles of two or three different sizes, irregularly distributed. 



In the Thraive Glen specimen which shows the free ends of the pleurae, the 

 pygidium is also partially preserved, and the two first pairs of pleurae composing 

 the lateral lobes are observed to have the same characters as the thoracic pleurae, 

 though their points are rather more strongly recurved. The third pair of pleurae 

 has its points almost parallel. The distinct border to the pygidium, shown in 

 Salter's figures of the type-specimen, is not present, the marginal free points 

 being in uninterrupted continuation of the pleurae, and they are, moreover, of 

 greater length than Salter indicates. In another smaller Thraive Glen specimen, 

 the axis is well preserved and is seen to possess three well-defined rings, and a 

 fourth less clearly marked off. In these features of the axis and of the 

 pleurae of the pygidium, the Girvan species agree more closely with Angelin's 

 L. sexspinus * than with Salter's typical form of L. laxatus, but Tornquist 2 and 

 Schmidt 3 consider L. sexspinus to be a synonym of L. laxatus, though the latter 

 author points out these differences. 



In the Thraive Glen specimen which possesses the thorax and pygidium, the 

 under surface of part of the head-shield with the hypostome in position is also seen. 

 The hypostome is large, subquadrate, with a broad flattened border having a 

 shallow rounded notch behind and slightly swollen at the sides of the central body 

 and in front of the posterior notch. The body is convex, arched in front, narrowing 

 and decreasing in height posteriorly, is strongly marked off from the border 

 laterally by wide shallow grooves and posteriorly by a weaker groove arched 

 gently forwards. Two short oblique lateral furrows indent the body near its base. 

 The anterior swollen part of the body is tuberculated in the same fashion as the 

 thorax, but the posterior part behind the lateral furrows is smooth. The border 

 has a few strong striae parallel to the margins. 



A fragmentary and poorly preserved cast of a glabella from Ardmillan in 

 Mrs. Gray's collection, may also belong to this species. The poorly preserved 

 specimen from Drummuck named " Lichas, sp. ind." by Nicholson and Etheridge 

 (op. cit.) is certainly attributable to this species. 



1 Angelin, ' Pal. Scand.,' p. 74, pi. xxxviii, figs. 7, 8. 



2 Tornquist, 'Undersokn. Siljausoru. Trilobitf.,' p. 31. 



3 Schmidt, ' Eev. Ostbalt. Silur. Trilob.,' pt. ii, p. 125. 



