ILL^ENUS. 73 



towards margins ; anterior outline regularly but slightly arched forward on each 

 side of axis, which itself projects to a similar extent ; no distinct fulcrum. Axis 

 narrow, short, about one fourth the width of the pygidium and less than half its 

 length, conical, with slight independent convexity but indistinctly defined by axial 

 furrows. Axial furrows very faint, shallow, converging posteriorly. Lateral lobes 

 without any furrows or ribs ; margin flattened. Whole surface of pygidium 

 covered by more or less continuous undulating striae, arched back and concentric 

 to margin posteriorly, but becoming straighter and more transverse towards 

 anterior margin of pygidium. 



Dimensions. — 



Length of pygidium .... 6*5 mm. 

 Width of „ . . . 11-5 „ 



,, of axis at front end . . 3'0 ,, 



Remarks. — The general shape of the pygidium, anterior outline, absence of 

 fulcrum, and size of axis appear to be similar to /. perovalis, Murchison, but the 

 conical shape of the axis (distinctly seen in our specimen, though the upper part is 

 chipped off) and particularly the striation over the whole surface, as in I.?puer, 

 Barrande, 1 seem to differentiate it from that species. 



Collection. — Mrs. Gray. 



Horizon and Locality. — Stinchar Limestone Group (Llandeilo) : Minuntion. 



1(5. Illaenus, sp. ind. Plate X, fig. 13. 



Description. — Two head-shields from Craighead in Mrs. Gray's collection show 

 certain peculiar features in the fixed cheeks not observed in any other Girvan 

 or, indeed, British specimens which I have seen. The head-shields are about half 

 as wide again as long and strongly convex from back to front, with a short broad 

 glabella faintly defined by almost obsolete axial furrows. The palpebral lobes 

 form prominent rounded subcylindrical lateral projections to the fixed cheeks, 

 such as are found in I. (Thaleops) ovatus, Conrad, 2 from the Trenton Limestone, 

 and in /. arcturus, Hall, 3 from the Chazy Limestone. Though the state of pre- 

 servation is not good in our specimens, they do not appear to have suffered 

 distortion. 



1 Barrande, ' Syst. Silur. Bolieme,' vol. i, Suppl., p. 73, pi. xiv, figs. 39 — 42 ; Novak, 'Zur Kennt. 

 bulim. Trilob.,' p. 36, pi. ix, figs. 1—3. 



2 Conrad, ' Proc. Acad. Nat. Sci. Pliilad.,' 1843, vol. i, p. 332; J. Clarke, "Lower Silur. Trilob. 

 Minn., Final Eep. " ; ' Geol. Nat, Hist. Surv. Minn.,' vol. iii, pt. 2 (1894), p. 716. 



3 Hall, ' Palseont. N. Y.,' vol. i (1847), p. 23, pi. iv bis, fig. 12. 



10 



