14 TRILOBITES OF GIRVAN. 



median post-axial lanceolate ridge on the pygidium, and the thorax is narrower 

 and especially the axis. 



"With regard to the head, the glabella of this Grirvan species has a rather 

 different shape to R. colhii and possesses traces of lateral furrows (though Port- 

 lock's type shows traces of them also), and the free cheek has the subgenal notch 

 in the posterior margin, which is a character of much importance and recalls the 

 angulation in R. (Teratorhynchus) hicornis, Reed.^ A similar feature is found in 

 the allied genus Apatocephalus {e. g. A. pecten,Wiman).- In some genera of the 

 Mesonacidge^ it is also met with, and therefore it is to be regarded as of a primitive 

 or reversionary nature. It appears from Walcott's researches (oj). cit.) that the 

 so-called " genal spine " in trilobites embraces structures belonging to at least two 

 distinct segments of the head-shield. 



With regard to foreign species we may note the resemblance of the pygidium 

 of B. nicliolsoni to that of U. latus (Olin-Wiman)'^ from the Chasmo])S Limestone of 

 Schonen. As Wiman remarks, this pygidium may be compared with Salter's 

 figure of B. dorsosjmiifer, which so closely resembles our Girvan form. 



There is one hypostome (PI. II, fig. 9) in Mrs. Gray's collection from the 

 Starfish Bed which must be referred to the genus Bemopleurides, and probably 

 belongs to this species. Both the cast and the impression of it were found together. 

 The description is as follows : — Shape transversely subelliptical, widest across 

 middle, with straight anterior and posterior margins and gently rounded sides ; 

 about one and a half times as wide as long; surface flattened. Body subreniform, 

 very slightly convex, with anterior edge bilobed and sharply notched in middle, 

 with traces of corresponding notch in posterior edge ; surface of body ornamented 

 with very delicate close concentric lineation. Anterior border narrow, depressed, 

 widening a little in middle at notch, and here provided with prominent triangular 

 tubercle. Anterior wings short, stout, bent up at right-angles, situated a little 

 behind anterior edge. Lateral borders narrow with thickened rim. Posterior 

 border projecting slightly at sides, arched up in middle, with faint low boss in 

 notch of body ; edge thickened ; marginal furrow strong, deep, 



Dimensions. — Length, 5*3 mm. ; maximum width, 6"8 mm. 



This small hypostome bears some resemblance to the one figured by Salter^ 

 without a specific reference, but has more rounded sides, a more regularly elliptical 

 body with median anterior and posterior indentations, and triangular tubercles in 

 them. 



Horizon and Locality. — Starfish Bed, Drummuck Group (U. Bala) : Thraive Glen. 



1 Eeed, ' Grirvau Trilobites,' pt. i, p. 3, pi. v, figs. 11—13. 



2 Wiman, ' ArMv f. Zool.,' vol. ii, no. 11 (1905), p. 6, pi. i, figs. 7—12. 



2 Walcott, " OleneJlns and other Genera of the Mesonacidie," ' Smithson. Instit. Misc. Coll.,' vol. 

 liii, no. 6 (1910), p. 23. 



■» Wiman, ' Bull. Geol. Instit. Upsala,' vol. viii (1907), p. 134, pi. viii, fig. 26. 

 5 Salter, 'Mem. Geol. Surv.,' dec. vii (1853), p. 9, pi. viii, fig. 5. 



