88 TRILOBITES OF GIRVAN. 



median tubercle. Narrow marginal border present round front of head-shield. 

 Fixed cheeks flattened, wider at their base than the neck of the glabella. A 

 strongly-curved furrow on each side marks off a small subeircular area near inner 

 basal angles of cheeks. Neck-segment marked off by strong furrow. Eyes situated 

 at level of second pair of glabellar furrows, and distant from axial furrows a little 

 more than half the width of neck of glabella. Facial sutures with anterior branch 

 cutting front margin of head-shield just outside lateral angle of frontal lobe, 

 running thence straight back to eye, behind which the posterior branch curves 

 sharply outwards and then runs back to cut posterior margin of head-shield. 

 Free cheeks unknown. Surface of head ornamented by fine curved lines 

 (indistinctly preserved) . 



Thorax of ten or eleven segments ; axis cylindrical, convex, not as wide as pleurae. 

 Pleurae simple, horizontal, flattened, with extra-fulcral portion deflected, wrinkled 

 transversely. Fulcrum moderate, situated at about half the length of pleurae. 



Pygidium of two shapes (probably of only sexual value), (1) transversely oval, 

 (2) elongated parabolic ; in both cases wider than long ; gently convex in middle, 

 but flattened and slightly concave towards margins. Anterior edge nearly 

 straight, with very weak fulcrum about halfway out. Axis short, conical, convex, 

 about a quarter the length of the pygidium or a little more, with sides slightly 

 arched and converging at about 45°; faintly trilobed with broader median conical 

 piece separated from narrow lateral pieces by faint longitudinal furrows. Whole 

 axis crossed by 7 — 8 parallel continuous transverse wrinkles. One strong ring on 

 front end of axis, marked off by a deep furrow. Axial furrows distinct. Lateral 

 lobes marked by seven pairs of nearly straight radiating raised and slightly 

 angulated ribs broadening and decreasing in height towards the margin, and 

 separated by strong interpleural furrows, which become weaker towards their 

 outer extremities. Seventh pair of pleurae separated by a slightly elevated and 

 angulated narrow post-axial piece, widening posteriorly, and nearly double the 

 width of any of the ribs. Surface of lateral lobes crossed by coarse concentric striae, 

 closely set and strongly undulating incrossing the ribs, especially near the margin. 



Dimensions. — 



Length of head-shield 



„ of glabella 

 Width of neck of glabella 



,, of frontal lobe of glabella . 

 „ of fixed cheek at base 



Length of pygidium . 

 Width of 

 Length of axis 

 Width of' 



. 8-0 



mm. (about). 



. 7-0 



33 33 



. 6-0 



,, 



. 10-0 



33 



. 5-0 



33 



Broad form. 



Long form. 



13'0 mm. 



16'5 mm. 



17"5 „ 



18-5 „ 



4-0 „ 



4-0 „ 



3-5 „ 



5-0 „ 



