25S BRITISH SILURIAN BRACHIOPODA. 



linear. Surface of both valves marked by numerous thread-like, radiating, angular radii, 

 which increase in number at various distances from the beaks by the means of bifurcation 

 as well as by the interpolation of shorter and smaller ribs between the larger ones, so that 

 three or four at times bifurcate. In the interior of the ventral valve the saucer-shaped 

 nmscular cavity is small. In the interior of the dorsal valve the cardinal process (not 

 tooth, as it has been often misnamed) is situated between two short brachial processes, 

 while the quadruple adductor scars are rather large and arranged in pairs, separated 

 longitudinally by a widish flattened ridge. 

 Length 7, width 9, depth 2 lines. 



Obs. Mr. Salter seems to have been the first to recognise that 0. lata and 0. 

 protensa. Sow., were one species, and this view was subsequently admitted by the gene- 

 rality of palaeontologists. Some authors have adopted the one, while others have pre- 

 ferred the other, designation ; but as that oi proiensa has been more generally made use 

 of, it will be here retained for the species. Although an abundant fossil in some localities, 

 it is found generally in the shape of distorted internal casts and external impressions, 

 the shell itself being rarely preserved ; indeed it is often difficult to make out its 

 exact shape, but when perfect it is more or less transversely semicircular, the valves 

 presenting but a very small degree of convexity. 



Position and Locality. 0. protensa is stated in ' Siluria ' to range from the Caradoc 

 into the Llandovery, but it is chiefly, if not exclusively, from the Lower and Upper 

 Llandovery that the shell has been procured. It occurs abundantly in the Lower 

 Llandovery at Goleugoed, Llandovery, Mandinam, Caernarvonshire ; also at Mathyrafal 

 south of Meifod and Pen-y-Craig. Prof. M'Coy quotes it from the Bala Schists of Cader 

 Dinmael, near Corwen, Denbighshire, also as very common in the Bala Limestone of Ash 

 Gill, Westmoreland, and at Llangynyw Rectory, near Welshpool, in Montgomeryshire. 

 Murchison mentions it likewise from Meadow Town, near Shelve, Salop. Mr. Hughes 

 has found the shell abundantly in the Upper Llandovery at Pentan and Iron Cold Brook, 

 Llandovery, also at Gorllvvyn Fach, Myddfai, in Wales. Mr. Salter got it from Wooltack 

 Park in the same formation. In the ' Synopsis of Sil. Fossils of Ireland ' Prof. M'Coy 

 quotes a variety (?) of this species from several localities in the Counties of Wexford and 

 Wicklow. 



Orthis TURGiDA, M'Coy. PL XXXII, figs. 12 — 20. 



Orthis TUKGiDA, il/'Coy. Aiuials Nat. Hist., 2iid ser., vol. viii, p. 399, 18.51; and 

 Brit. Pal. Foss. in Canib. Mus., p. 229, pi. in, figs. 20—24, 18.52, 



— — Id. Siluria, 4th ed., p. 527, 1867. 



— — Salter. Memoirs of the Geological Surve}', vol. iii, pp. 258, 268, 276, 



1866. 



