42 
SILURIAN 
ORTHIS TIBETICA. 
Plate 4, fig. 5 (4, junior ?). 
O. longa, semiuncialis, utrinque convexa, subcarinata, striata. 
Talva ventralis convexior, nate prominula (areaque magna), per medi¬ 
um subcarinata ; striata, stria centrali prominula, reliquis fasciculatis. 
Vulva dorsalis, praesertim ad unbonem convexa, area centrali elevata, 
per medium planatos-ulcata, lateribus declivibus. Striae incrementi 
inconspicuae. 
A shell which at first sight much resembles the 0. testu- 
dinaria from the Caradoc rocks of Britain. It is rounded, 
with a hinge line rather shorter than the width of the shell. 
The ventral and dorsal valves are both convex, and almost as 
long as wide. The length is eight lines; the width seven 
lines. The beak of the ventral valve a is a good deal raised; 
and the area is broad for this group of Orthides. Its con¬ 
vexity is not very great, but it rises into a low earina along 
the centre, which earina is crowned by one of the principal 
ribs. 
Internally, the muscular impressions of the valve are cir¬ 
cumscribed, occupy rather a small space, and are not bilobed. 
The dorsal valve b is also convex near the beak, but is 
thence flattened down the middle, and the flattening increased 
till a broad shallow groove, with a decided elevation on either 
side of it, marks out the central area. The sides or lateral 
areas slope away from this central portion, and no part of the 
shell is truly flat. In the variety figured at c, which appears 
also to be distorted by pressure, the central furrow is broader • 
it is triangular in both cases. 
The strhe in either valve are fasciculate; the larger ridges, 
which reach the beak, being grouped with and interlined by 
numerous smaller ones, so as to produce a very unequal surface. 
None, however, is very prominent, but the central one in the 
ventral valve. This is always, as above stated, conspicuous, 
and in one specimen (fig. d) two others are equally prominent. 
The lines of growth are very obscure. Near the margin the 
shell is a little antiquated. 
