In cases of doubt, evidence may be obtained from traces of organic structure, from the presence of carbonaceous matter, or, as Zeiller has pointed out, by the remains of animals such as Bryozoa being attached to the cast, showing that it represents a solid body and not a mere cavity or furrow.
The Coal Measures are covered by marine shales with numerous bryozoa; and, on the horizon of the Greta Coal Measures of New South Wales, is a bed of Carboniferous glacial deposits.
At one time they were referred by some to the Polyzoa (Bryozoa), and later, by almost general consent, to the Hydroida (Calyptoblastea) among the Hydrozoa (Hydromedusae).
The Bryozoa were also abundantinsomeregions (Polypora, Fenestella), including the remarkable form known as Archimedes.
The Polyzoa or Bryozoa, which has been both added to and again removed from the Mollusca between Cuvier's date and the present day.