208 BRITISH SILURIAN BRACHIOPODA. 
have recognised in Gothland two varieties of this shell, but all those hitherto found in our 
British Silurian rocks belong to one of these only. In his description of Orthis bzloba, 
or Spirifer cardiospermiformis (as he terms it), Baron von Buch compares the diverging 
free lobes with what we find in some specimens of Terebratula diphya; but the last-named 
shell is a true Zeredratula, provided with a short loop, somewhat similar in shape to 
what we see in Zered. vitrea or T. carnea ; still, a similar cause may, in both cases, 
easily have occasioned the deviation of the lobes, although both species and genera are 
distinct. 
Position and Locality. Orthis biloba seems to have enjoyed a somewhat extended 
geological vertical range, for we find it in the Caradoc-Bala, Lower and Upper Llandovery, 
Woolhope, Wenlock, and Lower Ludlow formations. In the Zower Ludlow it occurs at 
Ledbury. In the Wenlock Limestone and Shales at Buildwas; Wenlock Edge; Lincoln 
Hill; Rock Farm; May Hill; Hay Head, near Walsall, Dudley, Worcester Beacon, &c. 
In the Woolhope beds at Storridge, in the Malvern district. In the Lower Llandovery 
at Mwmffre, Llandovery. In the Caradoc-Bala at Cefn Rhyddan; Glyn Ceiriog, 
Denbighshire, &c. &c. 
In Scotland it is not rare in the Wenlock Shales of the Pentland Hills. I am not 
acquainted with any Irish specimens. 
Abroad it abounds in the Silurian rocks of the Island of Gothland, and is also found 
in Norway; but it is not known to me from Russia. It abounds in the Niagara group 
at Wolcott and Rochester, more rarely at Lockport, in the State of New York, &c. 
Ortuis Lewis, Dav. Pl. XXVI, figs. 4 to 9. 
Orruis Lewisi1, Dav. Bull. Soc. Géol. France, 2nd ser., vol. v, p. 323, pl. iii, 
fig. 19, 1848. 
a — Morris. Catalogue of British Fossils, p. 140, 1854. 
— — Salter. Siluria, 2nd ed., p. 544, 1859. 
— — Lindstrom. Gotlands Brachiopoder, Ofv. K. Akad. Forhandl., p. 370, 
1860. 
— — Dav. Trans. Geol. Soc. of Glasgow, Pal. Series, vol. i, pl. ii, figs. 5, 
6, 7, 1868. 
Spec. Char. Shell small, somewhat pentagonal, about as wide as long, tapering at the 
beak, slightly indented in front, rounded laterally. Hinge-line straight, usually slightly 
shorter than the width of the shell. Dorsal valve semicircular, moderately convex, divided 
into two lobes by a median longitudinal depression or sinus ; hinge-area triangular, narrow. 
Ventral valve convex, deeper than the opposite one, somewhat raised or keeled along the 
middle; beak produced, angular, slightly incurved ; area large, triangular, fissure open. 
Surface of valves marked by about twenty-eight thread-like radiating ribs, with inter- 
