36 TRILOBITES OF GIRVAN. 



Pygidium small, with broad blunt axis of two segments, each bearing three 

 equidistant tubercles ; pleural lobes flat, bearing a few large tubercles ; margin pro- 

 vided with five pairs of delicate long radiating denticulated spines and a median 

 one, all of the same length and equidistant. Width of thorax about 2-5 mm. 



BemarJcs. — The above description is based on one minute incomplete specimen 

 showing three or four thoracic segments with the pygidium attached, all more or 

 less imperfect. But the characters are so interesting and unusual that it deserves 

 a specific designation. Its affinities are plainly with A. verneuili, Barrande,^ and 

 A. mira, Barrande^; the. double spines of the thoracic pleurge of the former species, 

 with the lower ones denticulated, the median pair of tubercles on the axial rings, 

 the short broad pleurge and the denticulated radiating spines round the margin of 

 the pygidium are features in common. The number, however, of the pygidial 

 spines is different, and they are longer in our form, but a more precise comparison 

 is impossible till better material is obtained. 



There is one small imperfect cranidium from Whitehouse Bay (PI. VI, figs. 

 5, 6), which may probably be referred to this species, but its poor condition 

 precludes a satisfactory or complete diagnosis. The cranidium is semi-circular ; 

 the glabella is semi-oval, with two pairs of subequal oval or subcircular lateral 

 lobes, completely circumscribed, and a sub-cylindrical median lobe narrowing a 

 little anteriorly to the first lateral lobes, and then suddenly expanding and slightly 

 overhanging laterally, with a rounded, not truncate, anterior end. 



The meso-occipital ring is very broad, being about one-third the length of the 

 whole head-shield, and is furnished on its posterior margin with a pair of lateral, 

 slightly divergent, horizontal spines, at least as long as the head-shield ; a strong 

 horizontal furrow marks off this ring, and on the surface of the latter are six 

 tubercles arranged in three pairs in two straight lines, the median anterior one 

 being the largest. A few large tubercles are present also on the glabella. The 

 eyes seem to be placed far back, and the roll-like rounded narrow fixed cheek and 

 ocular ridge appear to be of equal size. 



Horizon and Locality. — 'Whitehouse Group (M. Bala) : Whitehouse Bay. 



Acidaspis terribilis, sp. nov. Plate VI, figs. 7 — 10, 



Specific Characters. — Head-shield transverse, about twice as wnde as long, 



moderately convex, arched down at sides ; anterior margin of cranidium slightly 



recessed owing to greater projection of free cheeks on each side. Glabella 



subovate, narrowing anteriorly, with strongly truncate front end and nearly 



straight sides. Median lobe subcylindrical, widening a little at level of second 



lateral furrows, and suddenly expanded in front into small lateral projections 



1 Barraude, ' Syst. Silur. Boheme,' vol. i, p. 710, pi. xxxviii, figs. 1 — 9. 

 Ibid., p. 735, p]. xxxix, figs. 1 — 11. 



