86 TRILOBITES OF GIRVAN. 



elongated genal spines longitudinally striated and of somewhat unusual length, as 

 they extend back past the thorax (against which they are closely pressed) to the 

 middle of the pygidium. Another specimen shows complete enrolment. 



This species has been previously recorded in the British Isles from the Stauro- 

 cephalus Limestone of Swindale, the Sholeshook Limestone of Sholeshook, and the 

 Keisley Limestone of Keisley. 



Collection. — Mrs. Gray. 



Horizon and Locality. — Drummuck Group (U. Bala) : Thraive Glen. 



Genus MENOCEPHALUS, Owen. 

 1. Menocephalus? (Tornquistia) cf. nicholsoni, Reed. Plate XIT, figs. 3 — 7. 



Remarks. — There are several specimens from Balclatchie in Mrs. Gray's 

 collection which bear a very close resemblance to the Keisley trilobite described by 

 the author * as Cyphaspis (Tornquistia) nirholsoni. The only point of difference in 

 the head-shield is that the median pre-glabellar furrow is obsolete or only represented 

 by a faint notch. In one specimen also there are slight traces of oblique basal 

 furrows on the glabella, obscurely marking off triangular basal lobes more than one 

 third the length of the glabella. The course of the facial suture is Avell shown, the 

 two branches making nearly a right angle at the eye, where the fine thread-like 

 ridge accompanying the suture is slightly thickened. 



In one specimen the thorax and pygidium are attached to the head-shield, and 

 though not quite perfect, and forced by crushing partly under the head-shield, show 

 certain distinctive features. Six or seven thoracic segments are present ; the axis is 

 cylindrical, strongly convex, and as wide as the pleura?. The pleuras are straight 

 and horizontally extended as far out as the fulcrum, which is situated at rather 

 more than half their length, and here they are bent downwards and slightly back- 

 wards. Each pleura is traversed by a submedian furrow, dividing it into an 

 anterior elevated portion which is ornamented with a row of 3 — 5 tubercles ; and 

 a posterior portion which is narrower, lower, and less conspicuously tuberculated. 

 The pygidium is short, transverse, broad, and twice as wide as long; the axis is 

 strongly convex and cylindrical, reaching the posterior border, and furnished with 

 three prominent rounded rings separated by deep transverse grooves; the lateral 

 lobes are wider than the axis, flattened, triangular, crossed by three rounded sub- 

 parallel ridges corresponding to the axial rings and finely tuberculated. The border 

 of pygidium is sharply marked off, wide, smooth, depressed and excavated. 



In addition to this complete individual there are several isolated head-shields 

 and pygidia from Balclatchie in Mrs. Gray's collection showing identical features. 



Affinities. — The close resemblance to Tornquist's Trilobites triradiatusin the case 

 1 Reed, ' Quart. Joura. Geol. Soc.,' vol. lii (1896), p. 433, pi. xxi, fig. 3. 



