248 BRITISH SILURIAN BRACHIOPODA. 
OrTHIS FLABELLULUM, J. de C. Sow. Pl. XXXIV, figs. 1—12 a. 
ORTHIS FLABELLULUM, J. de C. Sowerby. Sil. Syst., pl. xix, fig. 8, and pl. xxi, fig. 8, 
1839. ‘ 
—  CALLIGRAMMa (pars), De Verneuil (not of Dalman). Geol. of Russia, vol. ii, 
p- 207, 1845. 
—  FLABELLULUM, M‘Coy. Synopsis Sil. Foss. Ireland, p. 30, 1846. 
— — Salter. Mem. Geol. Survey, vol. ii, p. 289, 1848, and vol. iii, 
p. 338, pl. xxi, figs. J—16, 1866. 
— — J. Hall. Pal. of New York, vol. ii, p. 254, pl. lii, fig. 6, 1851. 
— — M‘Coy Brit. Pal. Foss., p. 218, 1852. 
— — Salter. Siluria, 2nd ed., p. 543, pl. v, fig. 12, 1859. 
Spec. Char. ‘Transversely semi-oval or slightly sub-quadrate, wider than long, 
broadest anteriorly ; sides and front rounded; hinge-line less than the width of shell. 
Ventral valve nearly flat, or slightly convex at the beak, widely depressed longitudinally 
between the beak and frontal margin; area rather narrow, often inclining backwards, 
with a triangular open fissure in the middle. Dorsal valve uniformly convex ; hinge- 
area narrow. Surface of valves covered by a variable number of radiating ribs, simple for 
some distance from the beaks, with concave interspaces of about equal width, but aug- 
menting in number from the margin by the interpolation of a smaller rib between each 
two of the longer ones, or by the bifurcation of some of the original ones, the whole 
surface being likewise marked by numerous small, equidistant, projecting, concentric lines 
of growth. In the interior of the dorsal valve the cardinal process is small, and situated 
between two moderately sized diverging cardinal plates; muscular scars separated 
into pairs by a wide, flattened, mesial ridge. In the ventral valve the muscular area is 
square, of moderate size, and with a deep indentation in front. In the ventral valve the 
external branches of the vascular trunks are turned outwards and backwards, sending off 
at imtervals longitudinal branches, which become divided several times as they near the 
margin. 
Length 12, breadth 15, depth 5 lines. 
Ods. Although my distinguished friend M. de Verneuil and some others have been 
tempted to regard Sowerby’s excellent species as a variety of O. calligramma, no two 
forms, when well understood, could be more clearly separate. In O. calligramma the 
ventral valve is regularly convex, in O. flabellulum it is flat or concavo-convex. In the 
first species the dorsal valve is less deep than the ventral, the reverse is the rule with 
that of Sowerby. Then the general shape of both shells is different, as well as that of 
their ribbing ; and there is also some modification of details in the interior of the valves 
of both forms. Prof. M‘Coy justly observes, “that the width of the ribs as well as the 
size, presence or absence, of the intermediary short ribs, varies much in different parts of 
