CHEIRURUS. 149 



than the others, curving strongly backwards and becoming weaker before uniting 

 with occipital furrow, so as to mark off transversely oval basal lobes, each more 

 than one third the basal width of the glabella. Occipital furrow well marked, 

 nearly straight, separating off prominent occipital segment, which is rounded and 

 produced behind medianly into a strong, curved, nuchal spine. Axial furrows deep. 

 Fixed cheeks small, short, narrow, less than one third the width of the glabella, 

 bent down on each side, with broad rounded border defined by strong marginal 

 furrow, and with genal angles furnished with long, rounded, divergent, tapering 

 spines, curved downwards like the nuchal spine. Facial sutures cutting outer 

 margin of head-shield at level of basal furrows of glabella and at right angles to 

 axial furrows. Anterior wing of fixed cheeks very narrow and flattened. Free 

 cheeks subtriangular ; eye small, prominent, conical ; border broad rounded. 

 Surface of head-shield and spines ornamented with coarse tubercles, intermixed 

 with smaller ones irregularly distributed. Extremities of spines longitudinally 

 striated. 



Axis of thorax broad, convex. First axial ring furnished with pair of small 

 lateral processes on anterior margin. Surface of ring divided into smooth, pos- 

 terior, semilunar portion (for enrolment), and raised anterior portion furnished 

 along its posterior edge with single row of small tubercles. Remainder of thorax 

 and pygidium unknown. 



Remarks. — The name Youngia was proposed by Lindstrom (op. cit., p. 50) in 

 1885 as a generic designation for this and two allied species. Unfortunately, our 

 knowledge of it is scanty, the material being imperfect or fragmentary. Nicholson 

 and Etheridge (op. cit., p. 105) do not give a very satisfactory account of the 

 specific characters, but I have now been able to add a description of the free 

 cheek and of a thoracic ring, as well as to supplement that of the glabella and 

 fixed cheek. Though this genus has so far only been found in the Silurian in 

 Scotland and Gotland, yet in the Urals Tschernyschew ' has described a species 

 (Y. uralica) from the Lower Devonian. 



Collections. — Mrs. Gray; Museum of Practical Geology; Hunterian Museum. 

 Glasgow. 



Horizon and Locality. — Penkill Group (Tarannon) : Penkill. 



7. Cheirurus, subgen. et sp. ind. (a). Plate XIX, fig. 13. 



From Dow Hill there are two small hypostomes in Mrs. Gray's collection which 

 may belong either to Cit. (Nieszh.) mdcus or to Ch. (Sphaero.) thomsoni, var. They 

 possess a convex elliptical body, separated by a deep groove from a narrow 

 rounded border embracing the sides and posterior end, nearly rectangular at 



' Tschernyschew, 'Mem. Com. Geol. St. Petersb.,' vol. iv, no. 3 (lS ( .Ki), pi. i, fig. 5. 



