54 TRIL0B1TES OF GIRVAN. 



rather wider ; its two posterior limiting furrows arch backwards and then forwards 

 in the middle where they meet. The median groove and tubercles are less distinct 

 than in the preceding segment. The fifth segment is marked off by a pair of 

 straight furrows not meeting in the centre. There is on this segment a median 

 ridge widening slightly behind, the surface of the segment being depressed as an 

 oval area on each side of the ridge. The neck segment is very narrow and linear, 

 with a straight posterior margin. 



Dimensions. — Length, C mm. ; width, 4 mm. 



Remarks. — This genus has been recorded by Mrs. Gray 1 from the beds of 

 Whitehouse Bay, and the above description is based on her specimens. 



The genus has not apparently been recorded hitherto from Britain. The 

 Bohemian forms 3 come from the horizon Etage E, associated with some of the 

 same species as here. 



Beecher 3 doubts if this genus can stand, for he believes it to represent the 

 glabella of an AUglina (= Cyclopi/ge) deprived of its eyes and free cheeks. No 

 further light is thrown on this point by these specimens. 



From Jemtland, Linnarsson ' records a species termed Boh.? denticulate-, and 

 Holm, following Beecher, assigns it to Mglina. 



Collection.— Mrs. Gray. 



Horizon and Locality. — Whitehouse Group (M. Bala) : Whitehouse Bay. 



Genus ILUENUS, Dalman. 



1. Illsenus aemulus, Salter, 1SG7. Plate VIII, fig. 5. 



1867. Illsenus {Dysplanus) aemulus, Salter, Mon. Brit. Trilob., p. 187, pi. xxviii, fig. 5. 

 1877. Illsenus aemulus, Woodward, Cat. Brit. Foss. Crust., p. 40. 



1882. Illsenus aemulus, Holm, Svensk. Artern. Illienus (Bill. k. Vet. Ak. Handl., vol. vii, No. 3), 

 p. 50. 



lie ma rls. — There is one perfect pygidium (together with the impression of it) 

 from Penkill in Mrs. Gray's collection, which agrees exactly with Salter's type 

 of the species. It shows the faint ribs and furrows on the lateral lobes which are 

 such a remarkable feature, though the arched broken fine lines on the surface are 

 not preserved. The submarginal fascia is well seen ; it is about half the width of 



1 'Mem. Geol. Surv.,' " Silur. Bocks Britain," vol. i, Scotland, 1899, pp. 517, 688. 



2 Barrande, ' Syst. Silur. Bolicme," vol. i, suppl., pi. xiv, figs. 30—32 (1872.) 

 s Beecher, ' Amer. G-eol.,' vol. xvii, 1896, p. 360. 



4 Linnarsson, ' Geol. Foren. Forh,' vol. ii, p. 495, pi. xxii, figs. 4, 5 ? 



5 Holm, ibid., vol. xix, pt. 3, No. 6 ; and ' Sv. Geol. Undersokn./ ser. C, No. 176, 1898, p. 11. 



