ILLSENUS P0RTL0CK1. 07 



Craigens. Saugh Hill Group (M. Llandovery): Woodland Point, PPenkill 

 Group (Tarannon) : Penkill. 



11. lllaenus portlocki, Salter, 1849. Plate IX, figs. 1 1 — 16; plate X, figs. 1, la. 



1843. Illsenus crassicauda, Portlock (non Dalman), Geol. Rep. Londond., p. 301, pi. x, figs. 7, 8. 



1849. Illsenus portlockii, Salter, Mem. Geol. Surv., dec. ii, art, ii, p. 3. 



1854. Illsenus portlockii, Morris, Cat. Brit. Foss, 2nd ed., p. 110. 



1867. Illsenus portlockii, Salter, Mon. Brit, Trilob., p. 197, pi. xxvi, figs. 3, 4. 



1877. Illsenus portlockii, Woodward, Cat. Brit. Foss. Crust., p. 42. 



1882. Illsenus portlockii, Holm, Svensk. Art. Illsenus (Bih. K. vet, Akad. Handl., vol. vii, No. 3), 

 p. 47. 



Specific Characters. — The thorax of this species has a broad axis, more than half 

 as wide again as the pleura?, gently convex and tapering very slowly to the 

 pygidium. The pleura? have the inner portion horizontally extended to the fulcrum, 

 which is situated about halfway out ; the extra-fulcral portion of the pleura? bends 

 down almost at right angles and at the same time backwards at about fc5° to the 

 inner portion in the front portion of the thorax, increasing to GO posteriorly. 



The pygidium is broad and short, from one and a half to twice as wide as long, 

 gently convex, bent down steeply behind and less so at the sides. The axis is broad, 

 half the width or rather more than half the width of the pygidium, with very slight 

 independent convexity in front, and very short, shallow, faintly impressed axial 

 furrows. The fulcrum is approximate, being distant from the axis about half or 

 rather less than half its width; the lateral angles are abruptly truncated at about 

 60° or more to the front edge. The caudal fascia is wide, marked by a few widely 

 spaced undulating stria? meeting in the middle line at an obtuse angle. The 

 surface of shell is smooth. 



Dimension*. — 





I. 



II. 



III. 



Length of pygidium 



12-0 mm. 



11-0 mm. 



6" 5 mm. 



Width of ,, (across broadest part) 



, 20-0 „ 



18-0 „ 



11-0 „ 



,, of axis 



. 9-0 „ 



8-0 „ 



5-0 „ 



Remark*. — There is a series of pygidia from Craighead, some with thoracic 

 rings attached, in Mrs. Gray's collection and the Museum of Practical Geology, 

 which are attributable to I. portlocki, Salter. I have had the opportunity of 

 examining Salter's types, through the kindness of .Mr. E. T. Newton, F.P.S., and I 

 do not therefore feel any doubt about the accuracy of the determination. There 

 are also some poorly preserved head-shields from Craighead which perhaps belong 

 to the same species, but the type is so distorted and imperfect that I hesitate to 

 identify them, as Salter's description alone is insufficient. 



