38 TRTLOBITES OF GIRVAN. 



from lateral angles of pygidium ; second pair large, stouter, fully twice the length 

 of p3'gidium, directed backwards, slightly curved inwards; six pairs of much 

 shorter subequal and equidistant spines behind second pair directed straight back- 

 wards from margin. (The third pair of spines is smaller than the rest, but this 

 part of margin is somewhat indistinct.) Axis broad, short, subquadrate, convex, 

 less than one-third width of pygidium and not touching border behind ; composed 

 of two segments with articulating ring at front end ; first segment consisting 

 of a rounded complete ring bearing a pair of stout tubercles; second segment 

 subquadrate, separated from first by pair of deep pits in shallow furrow and 

 bearing a pair of small tubercles. Axial furrows with pair of deep pits in them 

 behind first segment. Lateral lobes crossed by rounded pleural ridge proceeding 

 outwards from first axial segment parallel to anterior edge of pygidium for three- 

 fourths its length, then curving d)ack suddenly at a point marked by a large 

 tubercle to pass into second marginal spine with slight basal swelling. Lateral 

 lobes behind pleural ridge ornamented with three to four large tubercles. Whole 

 surface of pygidium granulated. 

 Diinensions. — 



Length of whole trilobite (without pygidial spines) 



Length of head-shield 



Width of „ . . 



Length of detached pygidium (without spines) 



Width of „ . . 



Width of axis of ,, 

 Bemarls. — One complete individual in an almost perfect condition (except the 

 pygidium) as an internal cast with its external impression, and one detached perfect 

 pygidium constitute the material from which the above description has been 

 drawn up. It is very doubtful if this species possessed eyes; but if they are present 

 they are very minute. A near ally is A. harrandei, Fletcher and Salter,^ of the 

 Wenlock Limestone ; the spinose ends of the pleurse and the spines on the axis are 

 almost identically developed ; the head-shield is very similar, but the glabella 

 and its lateral lobes are better defined and the axial furrows more distinct. A. 

 verneuili, Barrande,- may also be compared, and with regard to the pygidium we 

 may specially draw attention to A. l-eijserliiKji, Barrande.'^ The head-shield of 

 A. jamesii^ also possesses many features in common. 



If we adopt the classification of Acidaspis proposed by Clarke and followed by 



Van Ingen,^ we must place .4. ferrihilis in the subgenus Ceratocepltala, AVarder, 1838. 



Horizon and Localitij. — Stariish Bed, Drummuck Group (U. Bala) : Thraive Glen. 



1 Lake, 'Quart. Journ. aeol. Soc.,' vol. lii (1896), p. 241, pi. viii, figs. 1, 2, 3 (?). 

 ~ Barraude, 'Syst. Silur. Bolieme,' vol, i, p. 710, pi. xxxviii, figs. 1 — 9. 

 3 Barraude, ibid., p. 708, pi. xxxvi, figs. 10—22. 

 * Salter, ' Mem. Geol. Suiv.,' dec. vii (1853), pi. vi, figs. 1, 1*. 

 ' 5 Van Ingen, ' School of Mines Quarterly,' vol. xxiii (1901), p. 38. 



. lG-5 



mm 



. 5-0 



5? 



. 9-5 



55 



. 4-0 



55 



. 11-0 



55 



. 3-0 



55 



