11. _Crania anomala_ Interior of dorsal valve, showing muscular
impressions and labial appendages.]
The soft body of the Brachiopod is in all cases protected by a shell
composed of two distinct valves; these valves are always, except in
cases of malformation, equal-sided, but not equivalved. The valves are,
consequently, essentially symmetrical, which is not the case with the
Lamellibranchiata,--so much so, that certain Brachiopod shells were
named _Lampades_, or lamp shells, by some early naturalists; but while
such may bear a kind of resemblance to an antique Etruscan lamp, by far
the larger number in no way resemble one. The shell is likewise most
beautiful in its endless shapes and variations. In some species it is
thin, semi-transparent and glassy, in others massive. Generally the
shell is from a quarter of an inch to about 4 in. in size, but in
certain species it attains nearly a foot in breadth by something less in
length, as is the case with _Productus giganteus_. The valves are also
in some species very unequal in their respective thickness, as may be
seen in _Productus_ (_Daviesiella_)[1] _llangollensis_, _Davidsonia
verneuilii_, &c., and while the space allotted to the animal is very
great in many species, as in _Terebratula sphaeroidalis_, it is very
small in others belonging to _Strophomena_, _Leptaena_, _Chonetes_, &c.
The ventral valve is usually the thickest, and in some forms is six or
seven times as great as the opposite one. The outer surface of many of
the species presents likewise the most exquisite sculpture, heightened
by brilliant shades, or spots of green, red, yellow and bluish black.
Traces of the original colour have also been preserved in some of the
fossil forms; radiating bands of a reddish tint have been often seen in
well-preserved examples of _Terebratula_ (_Dielasma_) _hastata_, _T_.
(_Dielasma_) _sacculus_, _T. communis_, _T. biplicata_, and of several
others. Some specimens of _T. carnea_ are of a beautiful pale pink
colour when first removed from their matrix, and E. Deslongchamps has
described the tint of several Jurassic species.
The valves are distinguished as _dorsal_ and _ventral_. The ventral
valve is usually the larger, and in many genera, such as _Terebratula_
and _Rhynchonella_, has a prominent beak or umbo, with a circular or
otherwise shaped foramen at or near its extremity, partly bounded by one
or two plates, termed a deltidium. Through the foramen passes a
peduncle, by which the animal is in many species attached to submarine
objects during at least a portion of its existence. Other forms show no
indication of ever having been attached, while some that had been moored
by means of a peduncle during the early portion of their existence have
become detached at a more advanced stage of life, the opening becoming
gradually cicatrized, as is so often seen in _Leptaena rhomboidalis_,
_Orthisina anomala_, &c. Lastly, some species adhere to submarine
objects by a larger or smaller portion of their ventral valve, as is the
case with many forms of _Crania_, _Thecidium_, _Davidsonia_, &c. Some
_Cranias_ are always attached by the whole surface of their lower or
ventral valve, which models itself and fills up all the projections or
depressions existing on either the rock, shell or coral to which it
adhered. These irregularities are likewise, at times, reproduced on the
upper or dorsal valve. Some species of _Strophalosia_ and _Productus_
seem also to have been moored during life to the sandy or muddy bottoms
on which they lived, by the means of tubular spines often of
considerable length. The interior of the shell varies very much
according to families and genera. On the inner surface of both valves
several well-defined muscular, vascular and ovarian impressions are
observable; they form either indentations of greater or less size and
depth, or occur as variously shaped projections. In the _Trimerellidae_,
for example, some of the muscles are attached to a massive or vaulted
platform situated in the medio-longitudinal region of the posterior half
or umbonal portion of both valves. In addition to these, there exists in
the interior of the _dorsal_ valve of some genera a variously modified,
thin, calcified, ribbon-shaped skeleton for the support of the ciliated
arms, and the form of this ribbon serves as one of the chief generic
characters of both recent and extinct forms. This brachial skeleton is
more developed in some genera than in others. In certain forms, as in
_Terebratula_ and _Terebratulina_, it is short and simple, and attached
to a small divided hinge-plate, the two riband-shaped lamina being bent
upwards in the middle (fig. 15). The cardinal 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_, =
muscles diducteurs principaux, _Gratiolet_); c', accessory
divaricators (muscles diducteurs accessoires, _Gratiolet_); b, ventral
adjuster (= ventral peduncular muscles, or muscles du pedoncule paire
superieure, _Gratiolet_); b', peduncular muscle.
13. _Magellania [Waldheimia] flavescens_. Interior of dorsal valve. c,
c', cardinal process; b', b', hinge-plate; s, dental sockets; l, loop;
q, crura; a, a', adductor impressions; c, accessory divaricator; b,
peduncle muscles; ss, septum.
14. _Magellania [Waldheimia] flavescens_. Longitudinal section of
valves. A, ventral, B, dorsal valves; l, loop; q, crura; ss, septum;
c, cardinal process.
15. _Terebratula (Liothyris) vitrea_. Interior of dorsal valve. l,
loop; b, hinge-plate; c, cardinal process.