=
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.

16. Loop of _Terebratulina caput serpentis_.

17. Longitudinal section of _Terebratella dorsata_. (References as in
fig. 14.)

18. Longitudinal section of _Magas pumilus_.]

[Illustration: FIG. 19.--_Magellania [Waldheimia] flavescens_. Interior
of dorsal valve, to show the position of the labial appendages. v,
Mouth. (A portion of the fringe of cirri is removed to show the brachial
membrane and a portion of the spiral extremities of the arms.)]

Each valve of the shell is lined by a mantle which contains
prolongations of the body cavity. The outer surfaces of the mantle
secrete the shell, which is of the nature of a cuticle impregnated by
calcareous salts. These often have the form of prisms of calcite
surrounded by a cuticular mesh work; the whole is nourished and kept
alive by processes, which in _Crania_ are branched; these perforate the
shell and permit the access of the coelomic fluid throughout its
substance. These canals are closed externally and are absent in
_Rhynchonella_, where the amount of calcareous deposit is small. In
_Lingula_ the shell is composed of alternate layers of chitin and of
phosphate of lime. The free edges of the mantle often bear chitinous
bristles or setae which project beyond the shell. As in the case of the
Lamellibranchiata, the shell of the adult is not a direct derivative of
the youngest shell of the larva. The young Brachiopod in all its species
is protected by an embryonic shell called the "protegulum," which
sometimes persists in the umbones of the adult shells but is more
usually worn off. In all species it has the same shape, a shape which
has been retained in the adult by the Lower Cambr