840 DR F. R. C. REED ON THE 



Portlock as O. grandis, and based on the internal cast of a brachial valve, was 

 referred, as above stated, by Davidson to 0. porcata ; but, while it agrees in internal 

 characters with M'Coy's form from Coniston and with our Girvan specimens, the ribs 

 appear to be much finer and more numerous, and the cardinal angles are more rect- 

 angular and the whole shell more subquadrate in shape, so that its specific separation 

 may be necessary. The interior of the pedicle-valve of the Tyrone shell was not 

 figured by Portlock, but Davidson figured (op. cit., pi. xxxi, fig. 20) one of his 

 specimens (now in the Jermyn Street Museum), and it agrees in the muscle-scar, etc., 

 with the Coniston O. porcata, but the valve is less concave and more subquadrate 

 and is rather abruptly deflected at the margin. A series of other examples from 

 Tyrone indicates that these features are constant. 



Our Girvan shell, on the other hand, closely resembles 0. Carleyi, Hall, # from the 

 Hudson River Group of America, and has frequently been identified with O. retrorsa, 

 Salter, which, as above stated, is placed by Davidson in O. porcata, M'Coy. Hall 

 and Clarke put both in the subgenus Plesiomys, but subsequently Schuchert f in- 

 cluded this subgenus in Diiiorthis, and the separation of the two seems inadvisable. 

 FoersteJ is inclined to separate 0. Carleyi from 0. retrorsa, but puts both in 

 Dinorthis. In the Girvan area 0. porcata appears to be confined to the Drummuck 

 Group, whatever its vertical range may be elsewhere. 



Horizon. — Drummuck Group (especially Starfish Bed). ' 



Locality. — Thraive Glen. 



Subgenus hebertella, Hall and Clarke. 



Orthis (Hebertella) balclatchiensis, Davidson. 



(Plate VII, figs. 13-20.) 



1888. Orthis Bouchardi, Davidson (pars), Mori. Brit. Foss. Brack., vol. iii, pt. vii, pi. xxvi, fig. 23 



(non figs. 16-22). 

 1883. Orthis batdetchiensis, Davidson, op. cit., vol. v, Silur. Suppl., p. 176, pi. xiii, figs. 12-14. 

 1883. Orthis crispa, Davidson (pars), ibid., p. 176, pi. xiii, fig. 8. 



Davidson's description of this species is correct so far as it goes, but the interior 

 of the shell is only figured without any remarks on its characters. Internal casts of 

 both valves from Balclatchie and Ardmillan in Mrs Gray's collection show that the 

 pedicle-valve had a strongly marked triangular muscle-scar, bluntly truncated w 

 front, and raised on a distinct thickening of the shell ; the diductor scars are narrow, 

 and are separated by a shorter adductor scar of equal width ; and the genital areas 

 are marked by radiating broken sinuous ridges and vascular sinuses, a specially 

 strong pair usually running forwards divergently from the front end of the diductor 

 scar and dying out at about two-thirds the length of the shell. 



In the brachial valve the crura are short, stout, and triangular ; there is a narrow 



* Hall and Clarke, Paleeont. New York, viii, pt. i (1882), pp. 197, 222, pi. va, figs. 14-16. 



f Schuchert, Bull. 87, U.S. Geol. Surv., 1897, pp. 134, 215. 



J Foerste, Ohio Naturalist, vol. xii, No. 3 (1912), pp. 449, 453, pi. xxii, figs. 8a-d. 





