ILLAENUS EXTENSUS. 61 



Remarks. — This .species appears to be represented at Craighead, but two 

 imperfect head-shields in Mrs. Grray's collection are the only evidence of it, The 

 forward position of the eyes and their relative size, the length of the axial 

 furrows and the course of the facial sutures appear to be identical with Salter's 

 species; hut the poorness of the material makes the identification somewhat 

 uncertain. Salter recorded this species from Craighead in 1848, and Armstrong 

 and Young from the same horizon at Aldons in 1876. 



( Collection. — Mrs. Gray. 



Horizon and Locality. — Stinchar Limestone Group (Llandeilo) : Craighead. 



6. Illaenus extensus, sp. nov. Plate IX, figs. 3 — 5. 



Specific Characters, — Head-shield broad, transversely semicircular, weakly con- 

 vex ; posterior margin nearly straight. Glabella slightly raised, narrow and short, 

 being in width less than one sixth that of the head-shield, and in length between 

 one third and one half that of the head-shield. Axial furrows well marked, slightly 

 concave outwards. At the base of the glabella a transverse furrow, strong at 

 the sides but weak in the centre, cuts off a rounded neck-ring, unusually wide for 

 Illaenus, and bearing a low median tubercle. Fixed cheeks slightly swollen and 

 rising up on each side of glabella to prominent eye-lobes situated at a distance 

 from the axial furrows rather less than the width of the glabella, and elevated 

 somewhat above it. Posterior wing of fixed cheeks small, narrow, steeply sloping 

 to posterior margin of head-shield. Facial sutures bend sharply outwards behind 

 eyes to cut posterior margin of head at very acute angle (15° — 20°), nearly halfway 

 out to genal angle. Anterior branch of facial sutures curves strongly outwards 

 from eye to anterior lateral margin with a general inclination of about 60 to 

 posterior margin of head-shield. Free cheek broad, almost an equilateral triangle 

 in general outline, the lateral and posterior margins meeting at about 60° at the 

 genal angle, which is produced into a short spine. Eye of moderate size, about 

 one sixth length of head, situated far back, not more than its own length from the 

 posterior margin of head. 



Thorax of nine segments. Axis sul (cylindrical, tapering very slightly to 

 pygidium, narrow (only about one fifth width of thorax), moderately convex. 

 Pleura? flat, horizontally extended and straight as far out as fulcrum, which is 

 weak and remote, being situated at more than half the length of pleura 1 . Beyond 

 fulcrum pleura 1 slightly bent backwards and more strongly downwards. Small 

 pit at base of each pleura in axial furrow. 



Pygidium large, semicircular, weakly convex, longer than thorax ; anterior v^^v 

 straight and horizontal as far out as fulcrum, beyond which it is obliquely truncated 



