24 TRILOBITES OF GIRVAN. 



Illaenus peachi, sp. nov. Plate IV, figs. 2, 3. 



Specific Characters. — Cranidium subquadrate, wider than long-, the length about 

 three-fourths the width ; posterior portion somewhat flattened, arched down gently 

 at sides and more so anteriorly, anterior margin gently rounded; eye-lobes, fixed 

 cheeks and glabella rise to same level posteriorly. Glabella about two-fifths the 

 length of head-shield, widest at base, narrowing towards front, but suddenly 

 expanding at front end, with decided independent convexity at base, dying out 

 anteriorly ; small, low median tubercle situated at about half its length. Axial 

 furrows deeply sunken, strong, convergent anteriorly for greater part of length, 

 but diverging suddenly at front end and terminating abruptly. Fixed cheeks convex, 

 rising steeply from posterior edge of head-shield to level of glabella ; eye-lobes 

 prominent, rounded, horizontal, sub-cylindrical, semi-circular in outline, in length 

 about one-fifth to one-sixth that of head-shield and situated less than their own 

 length from posterior margin. 



Byes pedunculate. Facial sutures curving gently outwards from anterior 

 margin of head-shield, then running back to eye-lobes sub-parallel, bending out 

 sharply behind eye to cut posterior margin acutely at about 30°. 

 Dimensions. — 



I ■ II 



Length of cranidium . . 26'0 mm. . (?) 30*0 mm. 



Width of ,, in front of eyes 32-0 „ . 39-0 „ 



,, ,5 across eyes . 36"0 ,, . 43*0 ,, 



Length of glabella . .II" 5,, . 15"0,, 



Width of „ at base . 16-5 „ . 20-5 „ 



,, ,, at narrowest part 12"0 ,, . 14"5 „ 



Bemarlis. — The shape of the glabella, the subcylindrical prominent eye-lobes, 

 and the course of the facial sutures are characteristic features of this species ; and 

 the posterior view of the head-shield is peculiar, the eye-lobes, fixed cheeks and 

 glabella lying at the same level owing to the steep rise of the cheeks from the 

 depressed posterior margin, which is also bent down sharply on each side. The 

 base of the glabella also is very convex, the axial furrows being here deeply sunken. 

 There are suggestions of a relationship to /. schmidti, Nieszk.,^ and /. ariensis, 

 Holm,^ but the elevation of the eye-lobes recalls I. tauricornis, Kut.^ We may, 

 however, specially draw attention to its probable affinity with I. angiisticollis, 

 Billings,* and I. conradi, Billings, as like these species it seems to lie between the 

 typical members of llldenvs and the subgenus Thaleops. 



1 Holm, ' Eev. Ostbalt. Silur. Trilob.,' pt. iii (1886), p. 107, pi. v, figs. 1—20. 



■^ Ibid., p. 80, pi. vii, figs. 1—3. 



3 Ibid., p. 74, pi. vi, figs. 1—11. 



* Eaymond, 'Ann. Carnegie Mus.,' vol. iv, nos. iii and iv (1908), p. 245, pi. ixi, figs. 1 — 5. 



