144 TRILOBITES OF GIRVAN. 



slightly divergent spines, twice or three times as long as the whole pygidium. 

 The first pair of pleurae broaden rapidly to the point where they begin to curve 

 "hack, beyond which they taper into the free spines; a short median furrow marks 

 their basal portion. The second pair of pleurae are not grooved, are very short 

 and broadly triangular, expanding very rapidly from their narrow base, and uniting* 

 behind the axis, so as to form the nearly straight posterior margin of the pygidium. 

 (It is probable that the triangular piece apparently belonging to the axis consists 

 of one axial ring and its corresponding pair of pleura? fused in the median line.) 

 Surface of pygidium tuberculated like head-shield. 



Dimensions. — 





I 1 



Length of head-shield 



? mm. 



Width ., 





17-0 .. 



Length ,, glabella 





7*5 ,. 



Width „ 



(across middle) 



6-0 .. 



Length ,, thorax 





8-0 „ 



Width „ 





12-5 „ I 



,, ,, axis 





2-0 „ 



Length ,, pygidium 



. (without spines) 



3-0 „ 



Width „ 





6-5 „ 



II 



8-0 mm 



24-0 „ 



10-0 „ 



8-5 „ 



10-5 „ 



24-0 „ 



5-0 „ 



(crushed) 



Remarks. — There has been much uncertainty and confusion about this species, 

 which was founded by Wyville Thomson primarily on some pleurae attached to an 

 imperfect pygidium, and was called Aridaspis nnica. These pleura? have a median 

 groove, in which lies a row of pits, as Salter correctly represented in his figure, 

 remarking in the description of the species that this was a character found in the 

 genus Eecoptocheile. Nicholson and Etheridge figured a number of specimens 

 under the name Staurocephalus ? aniens (M., fasc. I, 1878, p. 118, pi. viii, figs. 9-16), 

 but only a few of them can be retained in this species (i.e. figs. 10, 11, von figs. 

 9, 12-10). The best figured specimen from Girvan (M., pi. viii, fig. 10) is much 

 more nearly perfect than the figure given by these authors indicates. In this speci- 

 men the base of the glabella is preserved, and it shows one pair of very oblique 

 basal furrows incompletely marking off triangular basal lobes. The glabella appears 

 to be of an oval shape. The eye is situated on the pitted cheek at the level of this 

 basal furrow and not far from the axial furrow. The genal angles are provided 

 with short spines, and the occipital segment is rounded and ornamented with a few 

 tubercles. The glabella is ornamented with somewhat sparse small tubercles. 

 The pygidium shows a pair of enormously-developed first pleurae, grooved near 

 their base ; the second pair of pleurae are very short, broad, triangular, expanded, 

 and abruptly truncated posteriorly, and meeting in the middle line behind the 

 pointed axis. The twelve thoracic rings are not very well shown in this specimen, 

 but fortunately another specimen of an individual, complete on one side, except 

 1 This is the specimen figured by Nicholson and Etheridge (op. cil., pi. viii, fig. 10). 



