12G TRILOBITES OF GIRVAN. 



the fulcrum by a slightly diagonal narrow furrow, situated nearer the anterior than 

 the posterior margin. (There is a fragment of an apparently similar thorax in 

 Mrs. Gray's collection from Craighead, and another from Balclatchie with one pair 

 of pleurae produced into very long spines.) These pygidia have the following 

 characters : Shape triangular, acutely pointed behind, and contracted posteriorly, 

 as broad (across the front) as long. Axis long, narrow, gently tapering, extending 

 two thirds the length of the pygidium, and with a width at front end less than one 

 third that of the pygidium ; possessing 20 — 25 incomplete narrow rings, leaving a 

 smooth central band down axis. Behind the axis and extremities of the pleurae is 

 a narrow tapering post-axial piece with a median ridge. Lateral lobes broad in 

 front and flattened horizontally, decreasing rapidly in width behind : each consisting 

 of four complete pleurae and one half pleura, successively decreasing in size and 

 strength of curvature, the anterior pair being very strongly curved ; the half 

 pleura is straight and lies along the side of the axis. Each pleura consists of a 

 large posterior raised, rounded ridge and a much narrower anterior ridge which 

 scarcely reaches the axis. The pleurae bear a few large tubercles. The posterior 

 ridge of the second pair of pleurae forms the greater part of the lateral margin 

 of the pygidium, and the extremities of the third pair bend out behind it at the 

 end of the axis to terminate in short, freely projecting points. 



These pygidia bear a general resemblance to that of G. bellatula, particularly in 

 the short axis and post-axial piece, as is well shown in Schmidt's figure (op. fit., pi. 

 xv, fig. 5), but the rings on the axis are much more numerous. 



The production of one pair of thoracic pleurae into longer spines than the rest, 

 and the enlargement of this thoracic segment, which are noticeable in some frag- 

 ments of a thorax from Balclatchie and Craighead, perhaps attributable to G. 

 bellatula, are met with also in G. loinchelli, Clarke, 1 from the Galena Limestone of 

 Minnesota, and in G. loveni, Linnarsson (see below). 



Collection. — Mrs. Gray. 



Horizons and Localities. — Balclatchie Group (Llandeilo) : Dow Hill ; Balclatchie. 

 (?) Stin char Limestone Group (Llandeilo) : Craighead. 



2. Cybele loveni, Linnarsson, 18G9, var. now girvanensis. Plate XVII, figs. 



1—4 



1876. Cybele rugosa, Armstrong and Young, Cat. West. Scot. Foss., p. lii. 



1878. Cybele rugosa, Nicholson and Etheiidge, Mon. Silnr. Foss. Girvan , fasc. i, p. 112, pi. xiv, 



tig. 13 ; pi. viii, figs. 5 — 7. 



1899. Cybele rugosa, Mem. Geol. Surv., Silur. Rocks Brit., vol. i, Scotland, pp. 672, 688. 



1 Clarke, ' Lower Silur. Trilob. Minnesota,' p. 747, fig. £9 (' Final Rep. Geol. Nat. Hist. Surv. 

 Minnes.,' vol iii, 1894). 



