218 BRITISH SILURIAN BRACHIOPODA. 
less prominently erect and very little incurved, so that the area of the larger valve is well 
exposed. In 1847 I confounded the shell under description with O. elegantula ; but, 
although then unacquainted with O. dasalis, I did not fail to remark that the Benthall 
Edge and the Falfield specimens seemed to differ materially from each other, and that it 
would not be improbable that O. ordicularis, Sow., might be one of the varieties of 
O. elegantula. his Falfield shell was consequently afterwards referred to O. orbicularis 
by myself and others; but this is a mistake, as the last-named shell is now affirmed to be 
the same as O. dunata, which differs in several particulars from O. dasalis, as will be found 
detailed under O. /umata. I am not yet quite satisfied that Dr. Lindstrém’s O. cana- 
liculata is well distinguished from O. dasalis ; at least our English Aymestry Limestone 
specimens of Lindstrém’s shell differ in little besides size from the shell found at Falfield, 
which is referred by Lindstrém to Dalman’s species. I will, however, provisionally 
retain them both as distinct, not having been able to compare with them a sufficient 
number of Swedish specimens. In fact, all these so termed species are so closely con- 
nected that it is hardly possible at times to feel quite certain to which should be referred 
some of the intermediate or less typical forms, and it may, perhaps, be found desirable 
hereafter to dispense with one or two of them. 
Position and Locality. The Wenlock Shale, Falfield, near Tortworth. It may 
occur in other localities, but they are unknown to me. Abroad it has been found in 
Gothland, Sweden. 
OrtHis CANALICULATA, Lindstrém. PI]. XXVII, figs. 12, 13. 
ORTHIS CANALICULATA, Lindstrém. Gotlands Brach., Ofv. K. Akad. Forhandl., p. 369, 
pl. xi, fig. 10, 1860. 
—  orpicuLaris, F. Schmidt. Beitrag zur Geologie der Insel Gotland, p. 44. 
Spec. Char. 'Transversely oval, broader than long, greatest breadth near the middle, 
slightly indented in front, broadly rounded laterally ; hinge-line shorter than breadth of 
shell. Dorsal valve semicircular, gently convex, most so at the umbone, flattened laterally 
close to hinge-line and cardinal extremities ; divided longitudinally by a moderately deep 
angular depression or sinus ; hinge-area narrow. Ventral valve moderately convex, most so 
near the beak, which is small and incurved ; area triangular, about one third as high as 
long. Surface of valves marked by numerous small, radiating, bifurcating, and interpolating 
ribs. In the interior of the dorsal valve the cardinal process is small, rhombic, undivided ; 
situated in the middle of the area, below the brachial processes are small but broad 
laminz, converging towards the cardinal process, while the quadruple muscular impressions 
are divided by a broadish, rounded mesial ridge. Length 6, width 7, depth 3 lines. 
