The numerous shells which belong to the extensive and cosmopolitan
genus _Orthis_ (fig. 50, a, b, c, and fig. 51, c and d),
are usually more or less transversely-oblong or subquadrate, the
two valves (as more or less in all the Brachiopods) of unequal
sizes, generally more or less convex, and marked with radiating
ribs or lines. The valves of the shell are united to one another
by teeth and sockets, and there is a straight hinge-line. The beaks
are also separated by a distinct space ("hinge-area"), formed in
part by each valve, which is perforated by a triangular opening,
through which, in the living condition, passed a muscular cord
attaching the shell to some foreign object. The genus _Strophomena_
(fig. 50, d, and 51, a and b) is very like _Orthis_ in
general character; but the shell is usually much flatter, one
or other valve often being concave, the hinge-line is longer,
and the aperture for the emission of the stalk of attachment is
partially closed by a calcareous plate. In _Leptoena_, again
(fig. 51, e), the shell is like _Strophomena_ in many respects,
but generally comparatively longer, often completely semicircular,
and having one valve convex and the other valve concave. Amongst
other genera of Brachiopods which are largely represented in the
Lower Silurian rocks may be mentioned _Lingula, Crania, Discina,
Trematis, Siphonotreta, Acrotreta, Rhynchonella_, and _Athyris_;
but none of these can claim the importance to which the three
previously-mentioned groups are entitled.
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