>Is there a structure which pushes up the spines<

The mantle tissue has to stick out as far as the spine while it is
forming.  Whether there is a permanent fold or projection or just a
temporary stretching will depend on the species and the spine.

> Is there a change in the aragonite crystal microstruture of the
spines compared with the rest of the shell?<

As a rule, they are similar to the microstructure of the shell
layers.  Depending on the spine, it may include all the shell layers
or just some of them.  There are some mollusks that have distinctive
structures as part of the sculpture, though I don't know that these
would necessarily qualify as spines.  For example, many bivalves have
little spicules or granules that are in the periostracum, often not
firmly attached to the shell.  These are microstructurally distinct
from the shell.

--
Dr. David Campbell
425 Scientific Collections Building
Department of Biological Sciences
Biodiversity and Systematics
University of Alabama, Box 870345
Tuscaloosa AL 35487-0345  USA

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