I 



II 



11'5 mm. 



16*0 mm 



3-5 „ 



o-O „ 



8-0 „ 



12-0 „ 



6-5 „ 



9-0 „ 



1-5 „ 



2-0 „ 



ACIDASPIS PLAYFAIRI. 35 



third width of pygidinm, convex, prominent, with obtuse end nearly reaching 

 border behind ; composed of two well-marked rings followed by pair of small 

 tubercles behind. Lateral lobes flat, horizontally extended; each obliquely 

 traversed by strong rounded pleural ridge running from first axial segment to 

 margin and produced into fourth marginal spines. Border of pygidium well 

 defined, gently raised ; margin provided with six pairs of spines ; posterior five 

 pairs of spines (second to sixth inclusive) nearly straight, sub-parallel, directed 

 backwards; first pair very small, short, slender, directed backwards; second and 

 third pairs larger, longer, subequal in size ; fourth pair the largest and longest 

 of all, more than twice as long as pygidium ; fifth and sixth pairs subequal, rather 

 longer than pygidium itself; base of fourth pair somewhat overlaps fifth pair. 

 Surface of pygidium and spines covered with rather sparsely distributed tubercles, 

 of subequal size. 



Dimensions. — 



Length of entire individual (without spines) 



„ of head-shield . 

 Width of 

 Length of thorax 



,, of pygidium (without spines) 



Remarks. — The material on which the above description is based consists of 

 the casts and impressions of several nearly perfect individuals, so that our 

 knowledge of this species is unusually full. The affinities of this species seem to 

 be with A. evoluta, Tornquist,^ of the Leptsena Limestone, and A. semievoluta, 

 Eeed,- of the Dufton Shales. A. Uughesi, Lake,^ is apparently also allied. It is 

 quite distinct from the other Grirvan species from the Starfish Bed, though 

 imperfect and poorly preserved isolated head-shields may be difficult to 

 distinguish. 



Horizon and Localiti/. — Starfish Bed, Drummuck Grroup (U. Bala) : Thraive Glen. 



Acidaspis playfairi, sp. nov. Plate VI, figs. 4 — 6. 



Specific Characters. — Thorax with broad cylindrical axis, one-third the width 

 of the thorax, and with each ring furnished with a median pair of large tubercles ; 

 pleurae short, broad, flattened horizontally, extended, with surface bearing a faint 

 broad submedian ridge and 2 — 3 large tubercles ; ends of pleurge abruptly truncated 

 and provided with very long slender spines, the posterior segment having the 

 spines almost parallel and projecting back behind pygidium ; with traces of 

 shorter straight horizontal denticulated spine lying at lower level. 



1 Tornquist, ' Siljansom. Trilobitf.' (' Sver. Geol. IJndersokii.,' Ser. C, No. 66, 1884), p. 28, pi. i, 

 figs. 24, 25. 



2 Eeed, ' Geol. Mag.' [5], vol. vii (1910), p. 214, pi. xvii, figs. 1—3. 



3 Lake, ' Quart. Journ. Geol. Soc.,' vol. lii (1896), p. 242, pi. viii, fig. 5. 



