846 DR F. R. C. REED ON THE 



But it is not the case that the Girvan Silurian ones have an odd number of ribs on 

 the fold and an even number in the sinus, as is found in the American species. 



It is probable that O. biforata also occurs at Shalloch Mill in the Whitehouse 

 Beds, but the specimens are poorly preserved. 



The numerous specimens from the Starfish Bed, Thraive Glen, exhibit considerable 

 variation amongst themselves. The fold is prominent, and has generally a rather 

 narrow crest and bears 2 or 3 strong angular plications, and there are 5-6 coarse 

 angular plications on each side, with 7-8 much smaller subequal ones near the 

 cardinal line. In shells of this type the sinus on the pedicle-valve is correspondingly 

 deep and narrow and holds 2-3 angular plications, and the lateral lobes have the 

 ribs developed as in the opposite valve. The whole shell is marked by its much 

 inflated character and strong fold and sinus, and more resembles the Silurian form 

 than the Craighead one. 



Horizons. — (l) Stinchar Limestone Group ; (2) Balclatchie Group ; (3) ? White- 

 house Group ; (4) Drummuck Group ; (5) Saugh Hill Group. 



Localities. — (l) Craighead; (2) Balclatchie, Ardmillan ; (3) Shalloch Mill; (4) 

 Thraive Glen, Auld Thorns ; (5) Woodland Point. 



Orthis (Platystrophia) dorsata (Hisinger) ? 

 (Plate VIII, fig. 25.) 



? 1837. Atrypa dorsata, Hisinger, Leth. Suec, p. 76, pi. xxi, fig. 14. 

 1883. Stricklandinial shalloehiensis, Davidson, Mon. Brit. Foss. Brack., vol. v, Silur. Suppl., p. 164. 

 pi. ix, figs. 26, 26a. 



The one specimen from Shalloch Mill on which Davidson based his Strichlandinia \ 

 shalloehiensis, and of which he gave a brief and inadequate description, appears to 

 have been quite misinterpreted. The larger valve which he described as the 

 " ventral " is probably the brachial valve, and only a small portion of the other valve 

 is preserved near the beak, but it is sufficient to show that this, the true ventral or 

 pedicle-valve, has a small, retrorse, low and not incurved beak, with a triangular hinge- 

 area set at right angles to the shell. The big swollen subcircular valve has, on the 

 other hand, a much elevated, swollen, large and strongly incurved beak rising well 

 above the hinge-line, with a very narrow hinge-area. The fold on this valve is low 

 but definite, rather narrow near the beak, but increasing gradually in width anteriorly ; 

 it bears 6-7 equal or subequal and subangular ribs, 3 or 4 of which start from the 

 beak, and 2 or 3 of them bifurcate at less than one-fourth their length. On each 

 lateral lobe of the valve are 9-10 similar equal subangular ribs, of which only 3-4 

 start from the beak, the others arising by division and the lateral ones curving back 

 slightly. The interspaces are equal to or rather wider than the ribs. 



This shell much resembles Orthis (Platystrophia) dorsata, Hisinger, from the 

 Chasmops Limestone, as figured and described by Lindstrom,* and it does not seem 



* Lindbtrom, Fragm. Silur., p. 28, t. xiv, tigs 33-37. 



