The specimens are all from the
Clinton Formation (May Hill Group) of Canada. (Original.)]
[Illustration: Fig. 67.--_Spirifera hysterica_. The right-hand
figure shows the interior of the dorsal valve with the calcareous
spires for the support of the arms.]
[Illustration: Fig. 68.--Upper Silurian Brachiopods. a a',
_Leptocoelia plano-convexa_, Clinton Group, America; b b',
_Rhynchonella neglecta_, Clinton Group, America; c, _Rhynchonella
cuneata_, Niagara Group, America, and Wenlock Group, Britain;
d d', _Orthis elelgantula_, Llandeilo to Ludlow, America and
Europe; e e', _Atrypa hemispherica_, Clinton Group, America, and
Llandovery and May Hill Groups, Britain; f f', _Atrypa congesta_,
Clinton Group, America; g g', _Orthis Davidsoni_, Clinton Group,
America. (After Hall, Billings, and the Author.)]
The Brachiopods continued to flourish during the Upper Silurian
Period in immense numbers and under a greatly increased variety
of forms. The three prominent Lower Silurian genera _Orthis,
Strophomena_, and _Leptoena_ are still well represented, though
they have lost their former preeminence. Amongst the numerous
types which have now come upon the scene for the first time,
or which have now a special development, are _Spirifera_ and
_Pentamerus_. In the first of these (fig. 69. b, c), one of
the valves of the shell (the dorsal) is furnished in its interior
with a pair of great calcareous spires, which served for the
support of the long and fringed fleshy processes or "arms" which
were attached to the sides of the mouth.
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