212 BRITISH SILURIAN BRACHIOPODA. 
regular in thickness, on account of the interpolation of one, two, or three smaller ribs 
between the principal ones. Some ribs also bifurcate. In the interior of the ventral valve 
a prominent tooth exists on either side of the base of fissure, supported by strong dental 
plates. The muscular saucer-shaped depression is divided longitudinally by a raised, wide, 
flattened ridge. In the interior of ventral valve a small bilobed cardinal process is situated 
between the prominent, diverging, curved brachial processes, on the outer side of which, 
close to the hinge-line, lies the deep-margined socket, while a widish rounded mesial ridge 
extends from under the cardinal process to about half the length of the valve, and sepa- 
rates into two pairs—the muscular (adductor) impressions. 
Length 9, width 8, depth 5 lines. 
Oés. This well-known and widely spread species varies slightly in its form, and itis, 
therefore, possible that one or two of the so-called species which will follow may be merely 
modifications or varieties of the species under description; but as palzontologists have 
maintained them as distinct, it will be preferable that for the present we should follow them in 
this particular. O. canalis of Sowerby is, however, an undoubted synomym, and it may be 
questionable whether the British specimens referred to O. parva are anything else than 
slight modifications in shape of Dalman’s species. At page 188 of the 2nd vol. of the 
‘Geol. of Russia,” M. de Verneuil informs us that O. parva is easily distinguished 
from O. dasalis, Dal., and O. orbicularis, J. Sow., by the strong incurvature of its beak. O. 
hybrida, of which the valves are subequal, and in which the front does not present any 
convex sinuosity towards the dorsal valve, cannot either be confounded with it. It is not, 
however, the same with O. elegantula, Dal.; the Russian species is so near to it that 
they have been united by Von Buch and D’EHichwald. Thus, O. elegantula has not 
its ribs fasciculated, nor are they angular, as are those of O. parva, &c. M. de Verneuil 
is, however, disposed to maintain them as distinct, on account of a supposed difference in 
the formation in which they occur, as well as on account of certain differences in the cha- 
racters of the shell. I have not, however, met with any true examples of O. parva in 
our British Silurian rocks ; but, on the contrary, have found well-characterised examples of 
O. elegantula in rocks of both Lower and Upper Silurian age ; and Mr. Salter informs me 
that the specimens in the Woodwardian Museum described and attributed by Prof. 
M‘Coy to O. parva are only slight modifications of O. elegantula. 
Position and Locality. O. elegantula occurs in the Llandeilo, Caradoc-Bala, Llan- 
dovery, Wenlock, and Ludlow formations. The localities in which it has been found 
are very numerous; we will enumerate some of the principal only. In the Ludlow 
north of Bringwood Chase, near Ludlow; Bodenham, Shucknall Hill, and in various 
places in the Woolhope, Usk, Builth, and Abberley districts ; at Freshwater Hast, Pem- 
brokeshire, &c. &c. In the Wenlock Limestone and Shale at Dudley; near Walsall; Bent- 
hall Edge, Lincoln Hill, Wenlock Edge, Buildwas (Shropshire). The Geological Surveyors 
found it at Craig-hir; Mwdwl Eithin ; Moel Seisiog; east of Merchlin ; Plas Madoc, 
North Wales; also in beds above the Denbighshire Grits at Bryn-mawr; Capel-y-rhiw ; 
