THE CARBONIFEROUS PERIOD. 
85 
Other species of Bivalves 
were also introduced, ap- 
proaching more nearly our 
Clams and Oysters, or, as 
they are called in scientific 
nomenclature, the Lamclli- 
branchiates. They differ 
from the Bracliiopods chiefly in the higher char- 
acter 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 res- 
piration in the Bracliiopods. We shall always 
find, that, in proportion as the functions are dis- 
tinct, and, as it were, individualized by having 
special organs appropriated to them, animals rise 
in the scale of structure. The next class of Mol- 
lusks, 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 de- 
scribed in a previous article, 
are now curled in a close 
coil, and the internal struc- 
ture of tlieir chambers har 
become more complicated. 
The suliebied wood-cut rep- 
