72 TRILOBITES OF GIRVAN. 



Dimensions. — 



Length of head (about ) . . . 15"0 mm. 



Width of head between eves . . .23-0,, 



„ of glabella . . . 10'5 ,, 



Length of eye . . . 5'0 „ 



Remarles. — These head-shields from Ardmillan are characterised by the posses- 

 sion of unusually large eyes close to the posterior margin, and a low median 

 tubercle at the base of the glabella. In one specimen the free cheeks are attached 

 but not well preserved. One detached free cheek, however, is fairly perfect. 

 The thorax and pygidium belonging to this form are unfortunately unknown. 



The species which bears the closest resemblance to this form appears to be 

 I. oculosus, Holm, 1 from Stage C of the Ordovician of the Baltic provinces. But 

 there is no median tubercle at the base of the glabella described in this Russian 

 form. The relative size and position of the eyes, course of the axial furrows and 

 shape of the head-shield appear to be identical. Salter's typical I. ocularis* from 

 the much higher horizon of the Chair of Kildare Limestone has the axial furrows 

 pursuing a similar course, but the eyes are too far forward and the genal angles 

 are produced. There is one free cheek, however, doubtfully assigned to /. ocularis 

 by Salter, 3 which in the genal angle and in the character of the doublure resembles 

 more closely our Ardmillan specimens. 



Collections. — Mrs. Gray; Museum of Practical Geology. 



Horizon and Locality. — Balclatchie Group (Llandeilo) : Ardmillan. 



15. Illaenu3 cf. perovalis, Murchison, 1830. Plate X, fig. 12. 



1831). Illxnus perovalis, Murcliison, Silur. Syst., p. 661, pi. xxiii, fig. 7. 

 1854. Illsenus perovalis, Murcliison, Siluria, 1st ed., pi. iv, figs. 13, 14. 

 Illxnus perovalis, Morris, Cat. Brit. Foss., 2nd ed., p. 110. 



1866. Ilhenus ptrocalis, Salter, Mem. Geol. Surv.. vol. iii, p. 256. 



1867. Illsenus (EctiV senus) perovalis, Salter, Mon. Brit. Tnlob., p. 211, pi. xxvi, figs. 5, 6 (? figs. 

 7,8). 



1877. Illxnus (Ectilhenus) perovalis, Woodward, Cat. Brit. Foss. Crust., p. 41. 



1882. Illxnus perovalis, Holm, Svensk. Art. Illsenus (Bill. K. vet. Akad. Handl., vol. vii, No. 3), 

 p. 47. 



Description. — One solitary pygidium in Mrs. Gray's collection from Minuntion 

 exhibits the following characters: — Shape subparabolic rather than semicircular, 

 not twice as broad as long, very slightly convex in middle and somewhat flattened 



1 Holm, 'Rev. Ostbalt. Silur. Trilob.,' pt. 3 (1886), p. 98, pi. xii, figs. 6—9. 



2 Salter, ' Mon. Brit. Trilob.,' p. 198, pi. xxix, figs. 7, 8. 



3 Ibid., pi. xxix, fig. 9. 



