6 TRILOBITES OF GIRVAN. 



similar elongated central tubercle or ridge traversing the two front rings ; and the 

 glabella also possesses a faint transverse furrow. The same features are notice- 

 able in A. sidenbladhi, Linnar., 1 while in A. caducus, Barr., 2 there is also a small 

 tubercle behind the transverse furrow on the glabella. 



Collections. — Mrs. Gray (f. M.) 3 ; Edinburgh Museum ; Museum of Practical 

 Geology, Jerniyn Street; Woodwardian Museum, Cambridge; Hunterian Museum, 

 Glasgow. 



Horizon and Localities. — Balclatchie Group (Llandeilo) : Balclatchie ; Ard- 

 niillan ; Dow Hill ; Penwhapple Burn. 



3. Agnostus perrugatus, Barrande, 1872. Plate I, figs. 5 — 7. 



1872. Agnostus perrugatus, Barrande, Svst. Silur. Boheuie, I suppl., p. 143, pi. xiv, figs. 14 — 16. 

 1880. Agnostus perrugatus ?, Nicholson and Etheridge, Mon. Silur. Foss. Girvau, fasc. iii, p. 296, 



pi. xx, figs. 6, 7. 

 1899. Agnostus perrugatus, Mem. Geol. Surv., Silur. Rocks Brit., vol. i, Scotland, pp. 517, 672, 688. 



Specific Characters. — Head-shield subquadrate, longer than broad, narrowing 

 slightly near base, feebly convex, with narrow concentric rounded border. 

 Glabella subcylindrical, but expanding at base, more than half the length and 

 about one third the width of the head-shield. Two oblique furrows cut off large 

 triangular basal lobes. Two transverse furrows angulated forwards in the middle 

 mark off a narrow transverse lobe behind the frontal lobe of the glabella. Frontal 

 lobe about one third the total length of glabella ; middle lobe shorter ; posterior 

 lobe the longest of all three. Axial furrows distinct, run convergently forward to 

 middle of glabella, then almost parallel to front end, round which they curve and 

 unite. Limb of head-shield marked on each side of glabella by G — 7 low rounded 

 radiating ridges, which bifurcate once or twice near border. In front of glabella 

 are two adjacent longitudinal similar ridges, which do not bifurcate but narrow 

 anteriorly. 



Pygidium of similar shape to head-shield, with narrow concentric rounded 

 border, widening a little posteriorly and furnished with a pair of short lateral 

 spines at posterior angles. Axis short, subcorneal, with rounded extremity; about 

 half the length of pygidium ; divided into three segments by transverse furrows. 

 Posterior segment the largest; the other two subequal, and interrupted down 



1 Linnarsson, ' Vestergotl. (Jamb. Silur. Aflagr.,' 1869, p. 82, pi. ii, figs. 60, 61. Brogger, 'Die 

 Silur. Etagen II and III,' 1882, p. 56. 



2 Barrande, ' Syst. Silur. Boh'cme,' I suppl., p. 142, pi. xiv, figs. 12, 13. Novak, ' Beitr. Palseont. 

 (Esterr.-Ung.,' iii, (2) ' Zur Kennt. bohm. Trilob.,' p. 56, pi. x, figs. 20 — 23. 



15 The Letters "f. M." in brackets after the name of any collection signify that the specimen or 

 specimens figured by Prof. Nicholson and Mr. Etheridge in their 'Monograph of the Silurian Fossils 

 <>i the Oirvan District ' are in that collection. 



