34 



it is a glabella or pygidium. The glabella measures one 

 inch long, the transverse measurement being the greater — 

 viz., one and a quarter inches. There is every reason to 

 believe that this glabella was fairly convex or gibbous, par- 

 ticularly about the centre. The axial grooves are wide 

 and deep (in the decorticated state), and wide apart, curving 

 slightly outwards, but not sufficiently to form a lobe as is 

 sometimes the case in this genus, such, for instance, as in 

 /. tauricornis, Kut.^ ; these grooves are also rather short, 

 extending but little beyond what would be the position of 

 the eyes were the latter preserved. The glabella narrows 

 towards the front, which is steep or highly inclined ; pos- 

 teriorly it slopes off gradually to the neck-furrow, and does 

 not overhang the latter as in some forms. The precise 

 course of the facial suture cannot be followed, from the 

 amount of lateral pressure the specimen has undergone, 

 and the same fact will no doubt to some extent affect the 

 degree of convexity of the glabella. On the side least 

 affected in this way, the facial suture seems to be fairly 

 straight and but little curved. 



The neck furrow, from glimpses of it that can be obtained 

 seems to have been very fine and narrow, and the neck- 

 lobe similarly so. In a few species of Illcenus^ such for 

 instance as /. revalensis, Holm,^ the glabella possesses 

 three oval depressions on each side, representing the gla- 

 bella furrows, but these have left no trace on our specimen. 

 The surface (i.e. decorticated) is devoid of sculpture, except 

 along the anterior margin, where a few parallel and semi- 

 anastomosing grooves are visible. 



I have never before had the good fortune to see an 

 Illcenus either from Australian or Tasmanian rocks, nor, 

 so far as I am aware, has the genus been described from 

 those of the latter Colony ; but De Koninck has recorded^ 

 the presence of /. WaJilenhergii from the Upper Silurian 

 of New South Wales, but without giving a figure. At the 

 same time there is a strong resemblance, in /. Johnstoni, 

 to this species, notwithstanding that the cephalic shield of 

 our fossil is imperfect, showing neither the eyes nor the 

 free cheeks. Under these circumstances the measurements 

 previously given can only be accepted provisionally. On 



» Holm, Mem. Acad. Imp. Sci. St. Petersbourg, 1886, XXXIIL, No. 8, 

 t. 6, f. 10, & 11a. 



^ Loc. lit. t. 2,f. 4 a &c b, 6 a Sc h. 



3 Foss. Pal. Nouv.-Galles du Sud, 1876, Pt. 1, p. 46. 



