" In the _Spirifers_, and in other ancient
genera, this apparatus is coiled up into a complicated spiral (fig.
67). It is these "arms," with or without the supporting loops or
spires, which serve as one of the special characters distinguishing
the _Brachiopods_ from the true Bivalves (_Lamellibranchiata_).]
[Illustration: Fig. 69.-a a', Meristella intermedia_, Niagara
Group, America; b, _Spirifera Niagarensis_, Niagara Group, America;
c c', _Spirifera crispa_, May Hill to Ludlow, Britain, and Niagara
Group, America; d, _Strophomena (Streptorhynchus) subplana_,
Niagara Group, America; e, _Meristella naviformis_, Niagara Group,
America; f, _Meristella cylindrica_, Niagara Group, America.
(After Hall, Billings, and the Author.)]
[Illustration: Fig. 70.--_Pentamerus Knightii_. Wenlock and Ludlow.
The right-hand figure shows the internal partitions of the shell.]
[Illustration: Fig. 71.--Upper Silurian Bivalves. A, _Cardiola
interrupta_, Wenlock and Ludlow; B, _Pterinea subfalcata_, Wenlock;
C, _Cardiola fibrosa_, Ludlow. (After Salter and M'Coy.)]
[Illustration: Fig. 72.--Upper Silurian Gasteropods. a, _Platyceras
ventricosum_, Lower Helderberg, America; b, _Euomphalus discors_,
Wenlock, Britain; c, _Holopella obsoleta_ Ludlow, Britain; d,
_Platyschisma helicites_, Upper Ludlow, Britain; e, _Holopella
gracilior_, Wenlock, Britain; f, _Platyceras multisinuatum_, Lower
Helderberg, America; g, _Holopea subconica_, Lower Helderberg,
America; h, h', _Platyostoma Niagarense_, Niagara Group, America.
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