832 DR F. R. C. REED ON THE 



valve and the less transverse outline of the shell and by the shape and characters of 

 the muscle-scars. The best preserved and most typical examples of O. ardmillanensis 

 come from Ardmillan. There is a variety (figs. 37-39) occurring at this locality and 

 in the Balclatch-ie conglomerate which seems to differ only in the greater number 

 (40-45) of its ribs, and probably it can only be separated as a variety of the type. 



Orthis (Plectorthis) duftonensis, Reed, var. 

 (Plate V, figs. 40, 41 ; Plate VI, figs. 1,2.) 



? 1883. Orthis calligramma, var. plicata, Davidson, Mon. Brit. Foss. Brack., vol. v, Silur. Suppl., p. 178, 

 pi. xiii, fig. 26 (excl. cet.). 



A few shells from Ardmillan showing the external impression and internal casts 

 of both valves seem to be specifically inseparable from 0. duftonensis, Reed,* 

 described from the neighbourhood of Melmerby. The shape, contour, and type of 

 ribbing are identical ; and the internal characters, including the remarkable vascular 

 sinuses in the pedicle-valve, agree in all essential particulars ; the number of ribs is 

 also relatively the same, varying from 40-45. In the typical 0. duftonensis from 

 Melmerby the ribs are rather narrower than the interspaces, but in the smaller 

 Ardmillan specimens the ribs are equal in width to the interspaces or rather wider 

 than them. The Girvan examples vary from 10-25 mm. in width. Davidson 

 labelled them O. calligramma, var. "plicata, and the general resemblance of O. duf- 

 tonensis to this form was pointed out by me in the original description of the species. 

 Vascular sinuses of much the same character occur in O. insculpta, Hall,t and in 

 O. Ranhini, Dav. (sens, restr.), as described below. 



Horizon. — Balclatchie Group. 



Locality. — Ardmillan. 



Orthis (Plectorthis) rustica, Sowerby. 

 (Plate VI, figs. 3-8.) 



1839. Orthis rustica, Sowerby, in Murchison's Silurian System, p. 624, pi. xii, fig. 9. 



1868. Orthis rustica, Sowerby, Davidson, Mon. Brit. Foss. Brack., vol. iii, p. 238, pl.xxxiv, figs. 13-17. 



1883. Orthis rustica, Sowerby, Davidson (pars), op\ cit., vol. v, Silur. Suppl., p. 226. 



1883. Orthis plicata, Sowerby, Davidson (pars), ibid., p. 226. 



The typical form of O. rustica occurs at Woodland Point, but is much less 

 abundant than its variety rigida. Externally the much closer and more Dumerous 

 ribs distinguish it from the latter as well as the more marked convexity of the 

 brachial valve which in var. rigida is nearly flat. But in some specimens the ribs 

 are fewer and mostly simple. Internally the muscle-scar in the pedicle-valve is, as 

 Davidson described, saucer-shaped or rather subcircular to subpentagonal, but it is 

 nearly as broad as long and not elongated, and is only about one-fourth the length 



* Reed, Geol. Mag., dec. v, vol. vii, 1910, p. 295, pi. xxiv, figs. 5-11. 

 t Hall and Clarke, op. cit., Brack, i, pi. va, fig. 13. 



