ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 829 



is much like 0. plicala, Sow.,* and the interior of the pedicle-valve and its shape 

 and contour somewhat resemble 0. virgata, Sow.,t but from the former it differs by- 

 its shorter hinge-line and from the latter by its different ribbing. O. sowerbyana, 

 Dav., is much more transverse and has more numerous and closer ribs. The interior 

 of our shell and general characters suggest that it is closely allied to 0. flabellites, 

 Foerste,j of the Clinton Beds. 



Orthis calligramma, Dalman, var. ? 



(Plate V, figs. 16-19.) 



? 1883. Orthis calligramma, Dalman, Davidson (pars), Mon. Brit. Foss. Brack., vol. v, Silur. Suppt., 

 pp. 181, 225. 



It is possible that some small shells, measuring 5-10 mm. in width, from the 

 Starfish Bed are referable to this species ; but as no larger examples have been found, 

 they may be only the young of some other species. In shape, ribbing, number of 

 ribs, and internal structure, so far as known, they possess the usual characters of this 

 species in its wide sense, and indeed more resemble the type-form than the Stinchar 

 Limestone and Mulloch Hill varieties above described. 



Horizon. — Drummuck Group (Starfish Bed). 



Locality. — Thraive Glen. 



Orthis Playfairi, sp. nov. 



(Plate V, figs. 20-32.) 



1883. Orthis calligramma, Dalman, Davidson (pars), Mon. Brit. Foss. Brack., vol. v, Silur. Suppl., 

 pi. xiii, fig. 24 (non cet.). 

 1 1883. Orthis unguis, Sovverby, Davidson, ibid., p. 117, pi. xiii, fig. 28. 



Shell subquadrate to subcircular, unequally biconvex to plano-convex ; hinge-line 

 rather less than maximum width of shell ; cardinal angles obtuse or slightly rounded. 



Pedicle-valve regularly convex, rounded ; beak more or less raised and swollen, 

 somewhat incurved, rising well above hinge-line ; hinge-area rather large, triangular, 

 lying in plane of valve or steeply inclined ; delthyrium wide, triangular. Brachial 

 valve flattened or very weakly convex, much less deep than opposite valve, with 

 weak median depression ; beak low, small, inconspicuous ; hinge-area narrower than 

 in opposite valve. 



Interior of pedicle- valve with large and stout teeth, and with elongated sub- 

 triangular muscle-scar occupying beak, bounded in front by short strongly convergent 

 ridges merging into low broad thickening of shell forming semioval boss ; diductors 

 long, subtriangular, separated by narrow adductor impressions ; ovarian areas marked 

 with weak radial orooves. 



Interior of brachial valve with very thick transverse cardinal plate bearing straight 



* Davidson, Mon. Brit. Foss. Brack., vol. iii, pi. xxxvii, fig. 1. t Ibid., pi. xxxvii, fig. 2. 



+ Grabau, Bull. N.Y. State Mus., No. 45, vol. ix, 1901, p. 186, fig. 94. 



