16(5 TRILOBITES OF GIRVAN. 



forward course for about one third the length of the head-shield, and then diverge 

 from each other strongly, curving outwards and being continued forwards to 

 within a short distance of the anterior margin to end in small pits. In front 

 of these pits the head-shield is marked with strong regular rugae and striae 

 (terrace-lines), concentric to the anterior margin. The axial furrows are of equal 

 strength along their whole course, and are well marked; but just behind their 

 nearest approach in their posterior course there is an oval expansion of the 

 furrows, forming a pair of special depressed areas a little in front of the eyes. 

 Owing to the course of the axial furrows, the glabella has an hour-glass shape, 

 its waist being only just half the width of its anterior end, while its base is about 

 a third wider than its waist. The glabella has no independent convexity except 

 behind its waist, where it is gently convex, and it is not defined at its anterior end. 

 There is a small median tubercle close to the base of the glabella. The neck 

 furrow is narrow and indistinct. 

 Dimensions. — 

 Length of head-shield . . . 14*50 mm. 



Width ,, between eyes . . K) - 50 „ 



Length of glabella . . . . 11-25 ;, 



Width „ at base . . . 9-00 „ 



,, ,, waist . . . 5 - 75 ,, 



front end. . . 12-00 „ 



Remarks. — The above described nearly complete head-shield (without the free 

 checks) from Dow Hill, shows some noticeable peculiarities which mark it off as a 

 distinct species. The posterior portion of a head-shield of the same species from 

 Ardraillan was previously described by the author (pp. cit. supra) as comparable 

 with /. oculosus, Holm, but it must now be removed from this association, as it is 

 certainly distinct from the other specimens referred to under that title. The 

 species which has been described from the Trenton and Chazy Limestones as 

 I. indeterminatus, Walcott, 1 much resembles our species in the peculiar course of 

 the axial furrows and long glabella, but it differs in the greater distance of the 

 eyes from the posterior margin. 7. ladogensis, Holm, 3 from the Ordovician (1> 3) 

 of the Baltic provinces, agrees likewise in the general development of the axial 

 furrows, but the eyes are smaller and more forward in position, and the glabella is 

 strongly swollen and rises much higher than the cheeks. The inflation of the 

 fixed cheeks between the glabella and the eye-lobe and the position of the latter 

 recall /. chudleighensis, liolm, :i but in other respects the head-shield is quite 

 different. 



1 Raymond, 'Ann. Carnegie Museum,' vol. iii, no. 2 (1005), p. 347, pi. 13, figs. 1, 2. 



2 Hulm, 'Eev. Ostbalt. Silur. Trilob.' ('Mem. Acad. Imp. Sci. St. Petersb.,' vol. xxxiii, no. 8, 

 L886), p. 113, pi. iii, figs. 5 a— e. 



•"• [bid., p. 101, pi. iii, figs. 1, 3,4. 



