Spirit Finds Another Shell-Like Object
One more piece that resembles a sea shell
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Spirit has many more findings now that show Gusev's lower layers to be identical to Meridiani in many ways- including geyser vents and what appear to be aquatic fossils.

Circled in red in the upper right section of the image you can see a strange convex curved object with a perfect lip around the lower edge.  This is identical to many sea shells found here on Earth.

The circular feature at lower left of center is where the instrument head was pressed against the soil

This is from Sol 843, panoramic imager.  Some images are not yet posted on the NASA/JPL site.  The original image is here at their site.
This is a false color magnification of the image using L2 and L7 data (not all images are on the NASA site yet).

I have cropped this image and you can see the shell-like object in the center.  It stands out against the others due to the curved lip.
This object is not a rock.  It is clearly of biological origin.  The lip is perfectly uniform in both width and spacing.  There are other features in the images from Sol 843 that also look very suspiciously like shells or shell fragments but this is the most striking of the group.

 
From Sol 848 I have constructed this false color stereo pair of the apparent shell, and it is consistent in all manners with a fragment of something similar to a helmet or harp shell.

Note at the bottom left end of the curved lip that it turn back toward the body of the piece, exactly as the lip of a helmet shell would.  Original image data is here at the NASA/JPL web site.
Just above this object and in the same image frame, we find this object which appears to be perhaps another portion of the same shell.  It is available in two views and I have constructed this stereo image of it as a consequence.

Object is rotated on its side 90 degrees for the stereo reconstruction.

 
This is a montage of some of the shell-like objects in these two frames of image data taken by Spirit on Sol 843.  A single instance might be dismissed as geology, but I have located and presented dozens of similar objects.

Combined with the fact that Mars was an ocean planet at one point in its history, we can safely conclude that these objects could be fossilized sea shells.