46 TRILOBITES OF GIRVAN. 



occurring in this bed to the species Ch. le'isleyeims^ which is found in the Upper 

 Bala (Keisley Limestone) of Keisley. The chief difference on which the latter 

 species was based lies in the characters of the pygidium, but there are certain 

 other distinguishing points in the head-shield, such as the squarer shape aiid greater 

 basal width of the glabella, the straighter and more horizontal course of the 

 lateral furrows, and the more forward position of the eyes, which are situated 

 rather in front of the second lateral furrows instead of behind them. These 

 features are present in the specimens from the Drummuck Beds, and thus separate 

 them from the true Silurian Ch. himucronatus, Murch. One example from the Star- 

 fish Bed is of a large size, the glabella measuring rather over 50 mm. in length. 



One fine hypostome from the same horizon may be attributed to the same 

 species. The description of it is as follows : Shape oviform, narrowing posteriorly ; 

 anterior end with somewhat flattened curvature, very slightly angulated in 

 middle; posterior end narrower, rounded. Body convex, most so at about half 

 its length, divided by lateral furrows into anterior subrhomboidal swollen larger 

 part and posterior less swollen crescentic smaller part. Lateral furrows situated 

 at about two-thirds the length of body, oblique, deep at sides, uniting across 

 middle as weak groove. Maculse prominent, large, pointed-oval, slightly oblique. 

 Border depressed, flattened, upturned, widening behind anterior lateral angles, 

 of almost uniform width behind lateral furrows ; marginal furrow well marked, 

 continuous. Anterior ears not preserved. Whole surface (including border) 

 ornamented with strong concentric equidistant regular striee, ending abruptly 

 against front edge of hypostome. 



Dimensions. — 



Length . . . . . 17'0 mm. 



Anterior width .... 13"5 ,, 



Width at maculge . . . . . 12-0 ,, 



Distance of maculge from front end . . 11"5 ,, 



In shape and general characters this hypostome may be compared with that of 

 a form referred by Salter^ to CA. hiiiiucronatiis from the Chair of Kildare. 



Cheirurus octolobatus, M'Coy. 



1906. Cheirur^is octolobatus, Reed, op. cit., pt. iii, p. 142. 



There appears to be no reason to hesitate in separating this species from 

 Ch. clavifrons, Dalman ; for, apart from the general characters of the head-shield, 

 there exist few points of similarity, and the thorax and pygidium are completely 

 different. A complete specimen of Ch. ocfolohntus from the Starfish Bed which 



1 Reed, 'Quart. Journ. Geol. Soc.,' vol. Iii (1896), p. 417, pi. xx, figs. 7—9. 



2 Salter, ' Mon. Brit. Trilob.,' pi. v, fig. 5. 



