148 
T. Davidson — What is a Brachiopod ? 
mainly of the brachial or labial appendages ; and so varied, yet con- 
stant in sbape to certain species is this laminal apophysis tbat it has 
served as one of the chief characters in tbe creation of botk recent 
and extinct genera (PI. IX. Figs. 2, 7, 9). The apophysis, or loop, 
is more or less developed, extending in some genera to upwards of 
three-fourths of the length of the shell, but in others it is so short 
and rounded as to project only a little beyond the hinge. In some 
genera it is attached only to the hinge-plate, as in Terebratula, Wald- 
Tieimia (PI. IX. Fig. 2), in others to a central longitudinal plate or 
septum 1 (PI. IX. Fig. 9). In certain families the apophysis presents 
the form of two spiral processes wkich nearly fill the interior of the 
shell, the ends of the spires being either directed outwards towards 
the Cardinal angles ( Spirifer , PI. VIII. Fig. 6), or placed horizontally 
with their apices directed inwards and towards the centre of the 
concave surface of the same valve, wliich they almost fill ; the inner 
sides of the spires are pressed together and flattened with their ter- 
minations close to each other near the centre of the bottom of the 
shell ( Atrypa , PI. VIII. Fig. 7). In the Rliynchoncllidce again it 
assumes the shape of two short, slender, curved laminm (PI. IX. 
Fig. 7) ; while in many genera and even families, Productidce, etc., 
there exists no calcified support for the labial appendages. 
The muscles, pallial sinuses, and ovaries, generally leave such in- 
dentations on tlie internal surface of the valves, in both recent and 
fossil genera and species, that they materially assist in determining 
the characters of extinct forms ; and in addition, the brachial or 
labial appendages often leave indications of their presence and cha- 
racter, which is also evinced by the shape of the calcified skeleton 
which supported them to a greater or lesser extent. 
Shell Structure. — The structure of the shell has been shown by 
Dr. Carpenter, Prof. King, Dr. Gratiolet and others to be generally 
distinct from that of the Lamellibranchiata or Gasteropoda. Dr. 
Carpenter, who has described the shell structure with infinite care 
and minuteness, informs us that in the shell of the Bracliiopoda, 
there is not that distinction between outer and inner layers, either in 
structure or mode of growth, which prevails among the ordinary 
bivalves ; and that it seems obvious, both from the nature and form 
of tbe shell substance, the mode in which it is extended, that the 
whole thickness of the Brachiopod shell corresponds with the outer 
layer only of the Lamellibranchiata. He adds that he has, however, 
occasionally met with a second layer in recent Terebratula;, within 
the earlier formed portion of the shell, but confined only to a part 
of the surface, instead of extending beyond it. In some families, 
according to Prof. King, it consists of three divisions ; the innermost 
1 In a very interesting paper on the development of the loop in Wald, cranium 
and W. septigera entitled “ Eidrag til Vestlandets Molluskfauna,” published in the 
“ Saerskilt Aftryk af Videnskabsselskabets Forhandlinger ” for 1875, Mr. Herman 
Frielc has started the hypothesis that the loop bccomes modified with age, and he 
indicates a most remarkable development of a simple loop out of a compound one, 
but before this important question can be definitely settled a complete connecting 
series of the different ages of a same species will have to be examined, which has not 
been hitherto done to my entire satisfaction. 
