The wood-cut below represents such
a shell, looked at from the side of the flat valve, showing the straight
cut of the line of juncture between the valves and the rising curve of
the opposite one, which looks like a hooked beak when seen in profile.
[Illustration]
Other species of Bivalves were also introduced, approaching more
nearly our Clams and Oysters, or, as they are called in scientific
nomenclature, the Lamellibranchiates. They differ from the Brachiopods
chiefly in the higher character of their breathing-apparatus; for they
have free gills, instead of the net-work of vessels on the lining skin
which serves as the organ of respiration in the Brachiopods. We shall
always find, that, in proportion as the functions are distinct, and, as
it were, individualized by having special organs appropriated to them,
animals rise in the scale of structure. The next class of Mollusks, the
Gasteropods, or Univalves, with spiral shells, were numerous, but,
from their brittle character, are seldom found in a good state of
preservation.
The Chambered Shells, or the Cephalopods, represented chiefly in the
earlier periods by the straight Orthoceratites described in a previous
article, are now curled in a close coil, and the internal structure
of their chambers has become more complicated.
Pages:
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258