rdinal process is prominent, and
on each side of the hinge-plate are situated the dental sockets; the
loop in _Terebratulina_ becomes annular in the adult by the union of its
crural processes (fig. 16). In _Magellania_ [_Waldheimia_] it is
elongated and reflected; the hinge-plate large, with four depressions,
under which originates a median septum, which extends more or less into
the interior of the shell (figs. 13 and 14). In _Terebratella_ the loop
is attached to the hinge-plate and to the septum (fig. 17). In
_Megerlia_ it is three times attached, first to the hinge-plate, and
then to the septum by processes from the diverging and reflected
positions of the loop. In _Magas_ the brachial skeleton is composed of
an elevated longitudinal septum reaching from one valve to the other, to
which are affixed two pairs of calcareous lamellae, the lower ones
riband-shaped; attached first to the hinge-plate, they afterwards
proceed by a gentle curve near to the anterior portion of the septum, to
the sides of which they are affixed; the second pair originate on both
sides of the upper edge of the septum, extending in the form of two
triangular anchor-shaped lamellae (fig. 18). In _Bouchardia_ the septum
only is furnished with two short anchor-shaped lamellae. Many more
modifications are observable in different groups of which the great
family _Terebratulidae_ is composed. In _Thecidium_ (figs. 3,4) the
interior of the dorsal valve is variously furrowed to receive the
lophophore folded in two or more lobes. In the family _Spiriferidae_
there are two conical spires directed outwards, and nearly filling the
cavity of the shell (fig. 5); while in _Atrypa_ the broad spirally
coiled lamellae are vertical, and directed toward the centre of the
dorsal valve. In the _Rhynchonellidae_ there are two short slender
curved laminae, while in many genera and even families, such as the
_Productidae, Strophomenidae, Lingulidae, Discinidae_, &c., there exists
no calcified support for the labial appendages. The ventral valve in
many of the genera is provided with two curved hinge-teeth, which fit
into corresponding sockets in the opposite valve, so that the valves
cannot be separated without breaking one of the teeth.

[Illustration: FIGS. 12-18.

12. _Magellania [Waldheimia] flavescens_. Interior of ventral valve.
f, foramen; d, deltidium; t, teeth; a, adductor impressions (=
occlusors, _Hancock_); c, divaricator (= cardinal muscles, _King_,