214 BRITISH SILURIAN BRACHIOPODA. 
it most approaches, by being nearly circular, and sometimes almost globular, both of its 
valves being often very convex ; the area is also curved, and not rectilinear, as in the last- 
named species; the muscular impressions, cardinal and _bracial processes, are also said to 
be slightly different. I must, however, confess that at times I find some difficulty in 
clearly discriminating certain minute differences which are said to occur in certain very 
closely related forms, and which are, perhaps, rather of varietal than specific value. 
Position and Locality. In England O. crassa occurs inthe Wenlock Limestone of the 
Malverns ; at Garcoed, near Usk ; and in the Lower Ludlow near Martley, on the road to 
Clifton, Abberley. Abroad it was found by Dr. Lindstrém in the “ Middle Gothland”’ 
group in the Island of Gothland. 
OrrHiIs HyBRIDA, Sow. Pl. XXVII. figs. 15, 16. 
ORTHIS HYBRIDA, Sow. Sil. Syst., pl. xiii, fig. 11, 1839. 
— — Portlock, Report on the Geol. of Londonderry, &c., p. 451, 1843. 
— — J. Hall. Geol. Rep. 4th District, p. 107, fig. 7, 1843. 
— — M‘Coy. Synopsis Sil. Foss., Ireland, p. 31, 1846. 
— — Dav. London Geol. Journal, pl. xiii, figs. 13, 14, 1847. 
— — De Verneuil. Bull. Soc. Géol. France, 2nd ser., vol. iv, p. 703, 1847. 
— _ Dav. et De Verneuil. Bull. Soc. Géol. de France, 2nd ser., vol. v, 
pp. 321 and 347, pl. iii, fig. 22, 1848. 
—- — Phillips and Salter. Mem. Geol. Survey, vol. ii, p. 289, 1848. 
— — Bronn. Index Pal., p. 855, 1848. 
_ — Barrande. Sil. Brach. of Bohemia; Naturw. Abhandl., vol. ii, p. 45, 
pl. xix., fig. 9, 1848. 
_— — Hall. Pal. New York, vol. ii, p. 253, pl. lii, fig. 4, 1852. 
_ _ M‘Coy. Brit. Pal. Foss., p. 220, 1852. 
— — F. Schmidt. Silurische Form. von Ehstland, &c.; Archiv Nat. Liy- 
Ehst- und Kurlands, p. 213, 1858. 
— — Salter. Siluria, 2nd ed., pl. xx, fig. 13, 1859. 
— _ Lindstrom. Gotlands Brachiopoder, p. 369, 1860. 
— — Salter. Mem. Geol. Survey, vol. ii, pp. 276, 279, 1866. 
Spec. Char. Rotundato-trigonal or lenticular, slightly wider than long, broadest 
posteriorly, straight or slightly indented in front; valves almost equally convex, most so 
near the beaks, flattened anteriorly. In the dorsal valve a slight depression extends 
sometimes all the way from the umbonal beak to the front; hinge-line very short, beaks 
approximate, areas small; surface of both valves closely covered with very fine, thread-like, 
radiating strize, increasing in number towards the margins from numerous interpolations, 
the valves being likewise crossed by concentric lines of growth. In the interior of the 
dorsal valve a small but prominent cardinal process is situated between two curved, very 
prominent, brachial processes, while a mesial ridge extends from under the cardinal process 
