ORDOVICIAN AND SILURIAN BRACHIOPODA OF THE GIRVAN DISTRICT. 845 



more incurved than M'Coy's figure indicates, while there is also a slight median 

 flattening or very shallow broad sinus in the anterior portion of the valve. The 

 brachial valve also shows a similar (though usually less marked) median depression. 

 The beaks of both valves are nearly of equal height, and the whole aspect of the shell 

 more recalls members of the subgenus Hebertella. The interior of a brachial valve 

 (fig. 23) probably referable to O. scotica, from Ardmillan, is in support of this view. 

 It seems more allied to 0. balclatchiensis than to 0. calligramma. 



Horizons. — (l) Stinchar Limestone Group ; (2) Balclatchie Group ? 



Localities. — (l) Craighead ; (2) Ardmillan, Balclatchie (conglom.). 



Subgenus platystrophia, King. 



Orthis (Platystrophia) biforata (Schlotheim). 



(Plate VIII, fig. 24.) 



1820. Terebratulites biforatus, Schlotheim, Petrefact., p. 265. 



1870. Orthis biforata, Schlotheim, Davidson, Mon. Brit. Foss. Brack., vol. iii, pt. vii, p. 268, 



pi. xxxviii, figs. 12—14. 

 1883. Orthis biforata, Schlotheim, Davidson (pars), op. cit., vol. v, Silur. Suppl., pp. 181, 225. 



Davidson figured this species from Craighead in the third volume of his Mono- 

 graph (loc. cit., supra), and in his Supplement likewise records it from this locality 

 and horizon, but from none other in the Girvan district. 



The Craighead form is characterised by having 4 ribs equidistant and of equal 

 size on the fold of the brachial valve and 3 in the corresponding sinus of the pedicle- 

 valve, all of which start from the beak and are usually equal, though sometimes the 

 middle one is larger ; there are also 7-9 (usually 8) ribs of equal size on each side. 

 The shells have rounded or obtuse cardinal angles, and the hinge-line is not produced, 

 being rather less than the width of the shell. Specimens from Balclatchie and 

 Ardmillan show the same features, but some examples from the last-mentioned locality 

 have only 5 or 6 ribs on each side, and these are larger and more rounded than in 

 those from Craighead. The shells from Dow Hill have 4 ribs in the sinus, the 2 

 middle ribs being also somewhat stronger, and there are 5 or 6 on the fold, with the 

 median one enlarged. Young shells from Craighead have only 2 ribs on the fold. 



These Lower Ordovician examples of O. biforata resemble O- (PI.) colbiensis, 

 Foerste, and its varieties as described by Foerste,* and are distinct from the Silurian 

 form, which occurs at Woodland Point in the Girvan area. This latter form has the 

 ribs much stronger and more angular, and the fold is not so much elevated ; there are 

 also only 6 ribs on each side and 1 smaller one close to the hinge-line. But the sinus 

 as in the Craighead ones, holds 3 equal ribs, and the fold bears 4 equal ribs. 



The chief and most striking difference is the greater strength and coarseness of 

 the ribs, and the shells recall 0. (PI.) prof undo sulcata, Meek, as figured by FoERSTE.t 



* Foerste, Bull. Denison Univ., vol. xvi, art. 2, p. 55, pi, iv, figs. 1-3. 

 f Foerste, op. cit., p. 58, pi. vi, figs. 15a, b. 



