Shell or Coral

[paul monfils]

I can't see an image if you posted one, but there are several species of hard bryozoans (microscopic colonial animals unrelated to corals) that frequently encrust small shells.  The aperture of the shell is usually visible, and may even be inhabited by a hermit crab, but the bryozoan colony may take on a very different shape from the original shape of the shell, often with spikes sticking out in various directions - or in just two directions, as in this one:

<!-- m --><a class="postlink" href="http://www.jaxshells.org/jdawley.htm">http://www.jaxshells.org/jdawley.htm</a><!-- m -->

.