844 DR F. R. 0. REED ON THE 



the Central Himalayas, but the internal characters so far as known are opposed to 

 referring it to the genus Scenidium. The species named O. equivocalis, Reed,* from 

 the Dufton Shales, seems to be allied or to belong to the same group. But we may 

 especially compare O. baldatchiensis, Dav., young examples of which are almost in- 

 distinguishable from it, both in shape, ribbing, and concentric imbrications. Indeed, 

 the specific separation of these species may be questioned, for no large specimens of 

 0. Lapworthi from the Balclatchie Beds are known, the largest measuring only 

 6'4 mm. in width. In young examples of 0. baldatchiensis the ribbing is much 

 coarser than in the adult, and most or all of the ribs are simple. Both species also 

 occur on the same horizon, at the same locality, and in the same state of preservation. 

 The Shalloch Mill specimens are doubtfully referred to the same species ; the median 

 sinus in the brachial valve seems deeper, more open and rounded, and the two ribs 

 in it smaller than the rest. One specimen with coarse lamellae (PI. VIII, fig. 18) is 

 probably distinct from the rest. 



Horizons. — (l) Balclatchie Group; (2) ? Whitehouse Group. 



Localities. — (l) Balclatchie (conglom.) ; (2) Shalloch Mill. 



Orthis (Hebertella) scotica, M'Coy. 



(Plate VIII, figs. 19-23.) 



1852. Orthis scotica, M'Coy, Syn. Pal. Foss. Woodio. Mus., p. 232, pi. iH, figs. 29, 29a, 296. 



1868. Orthis call/ gramma, var. scotica, M'Coy, Davidson, Mori. Brit. Foss. Brack., vol. iii, p. 244, 



pi. xxxv, fig. 20 (121, 22) (figures incorrect). 

 1883. Orthis calligramma, var. scotica, M'Coy, Davidson, op. cit., vol. v, Silur. Suppl., p. 226. 



The type-specimen from Craighead is in the Sedgwick Museum, and, though 

 somewhat rubbed, shows the characters well. There are several other examples of 

 this form from Craighead in Mrs Gray's and other collections which Davidson 

 labelled O. calligramma, as he considered it only a variety of the latter. It differs* 

 however, in shape, convexity, size of beaks, and especially in ribbing from this 

 species, so that it seems necessary to regard it as distinct. It is easily separated 

 by its exterior from the variety of 0. calligramma and 0. Playfairi, which occur with 

 it at Craighead. Typically the shell is transversely subquadrate to semielliptical, 

 the length being to the breadth from 3:4 to 2:3, and it is moderately and sub- 

 equally biconvex, the valves being nearly of the same depth ; the cardinal angles are 

 nearly rectangular or obtuse, and the hinge-line is nearly equal to the width of the 

 shell. The ribs are rounded, equal or subequal in strength on the margin, about 

 28-3G in number, and every other one is intercalated close to the beak with con- 

 siderable regularity ; the interspaces are equal to or rather less than the ribs in 

 width. Thus the ribbing is markedly different from that of 0. Playfairi. With 

 regard to the contour of the valves, the pedicle-valve in 0. scotica has not a much 

 higher beak or hinge-area than the brachial valve, and it is less conical and the beak 



Rked, Geol. Mag., dec. v, vol. vii (1910), p. 297, pi. xxiv, figs. 1-4. 



