Blastoids are primitive echinoderms that are related to sea urchins, crinoids, and sand dollars.  On Earth, some blastoids are known as pentremites based on their five-fold symmetry.  On Mars, some urchins display six-fold symmetry while others are more typical and have five-fold symmetry.  This seems to hold for Martian blastoids as well, since some appear to have a six-fold symmetry.

Here is a catalog of Martian rind objects, the sol and imager used to capture the data, and the link to the original file where I located them.

Information about this and many other Martian fossils is available in the new book "A Fossil Hunter's Guide to Mars" on sale here.

		Rover: Opportunity    Sol: 056    Instrument: Microscopic imager    Filename:1M133157669EFF06HEP2956M2M1.JPG Link: http://marsrovers.jpl.nasa.gov/gallery/all/1/m/056/1M133157669EFF06HEP2956M2M1.JPG
		Rover: Opportunity    Sol: 541    Instrument: Panoramic imager    Filename:1P176210806EFF57MJP2571L6M1.JPG Link: http://marsrovers.jpl.nasa.gov/gallery/all/1/p/541/1P176210806EFF57MJP2571L6M1.JPG
		Rover: Opportunity    Sol: 543    Instrument: Panoramic imager    Filename:1P176393136EFF5842P2351L5M1.JPG Link: http://marsrovers.jpl.nasa.gov/gallery/all/1/p/543/1P176393136EFF5842P2351L5M1.JPG
		Rover: Opportunity    Sol: 591    Instrument: Panoramic imager    Filename:1P180653591EFF60Y5P2380L5M1.JPG Link: http://marsrovers.jpl.nasa.gov/gallery/all/1/p/591/1P180653591EFF60Y5P2380L5M1.JPG
		Rover: Opportunity    Sol: 591    Instrument: Panoramic imager    Filename:1P180653591EFF60Y5P2380L5M1.JPG Link: http://marsrovers.jpl.nasa.gov/gallery/all/1/p/591/1P180653591EFF60Y5P2380L5M1.JPG
		Rover: Opportunity    Sol: 614    Instrument: Panoramic imager    Filename:1P182693334ESF62JLP2543L7M1.JPG Link: http://marsrovers.jpl.nasa.gov/gallery/all/1/p/614/1P182693334ESF62JLP2543L7M1.JPG
		Rover: Opportunity    Sol: 614    Instrument: Panoramic imager    Filename:1P182695514ESF62JLP2546R1M1.JPG Link: http://marsrovers.jpl.nasa.gov/gallery/all/1/p/614/1P182695514ESF62JLP2546R1M1.JPG
		Rover: Opportunity    Sol: 649    Instrument: Panoramic imager    Filename:1P185802415EFF64EZP2571L7M1.JPG Link: http://marsrovers.jpl.nasa.gov/gallery/all/1/p/649/1P185802415EFF64EZP2571L7M1.JPG
		Rover: Opportunity    Sol: 650    Instrument: Panoramic imager    Filename:1P185887104EFF64I6P2571L7M1.JPG Link: http://marsrovers.jpl.nasa.gov/gallery/all/1/p/650/1P185887104EFF64I6P2571L7M1.JPG
		Rover: Opportunity    Sol: 660    Instrument: Panoramic imager    Filename:1P186777735EFF64KCP2291L2M1.JPG Link: http://marsrovers.jpl.nasa.gov/gallery/all/1/p/660/1P186777735EFF64KCP2291L2M1.JPG
		Rover: Opportunity    Sol: 706    Instrument: Panoramic imager    Filename:1P190874303EFF64KCP2439L7M1.JPG Link: http://marsrovers.jpl.nasa.gov/gallery/all/1/p/706/1P190874303EFF64KCP2439L7M1.JPG
		Rover: Opportunity    Sol: 719    Instrument: Panoramic imager    Filename:1P192005640EFF64KWP2441L7M1.JPG Link: http://marsrovers.jpl.nasa.gov/gallery/all/1/p/719/1P192005640EFF64KWP2441L7M1.JPG
		Rover: Opportunity    Sol: 973    Instrument: Panoramic imager    Filename:1P214568457EFF76POP2403L5M1.JPG Link: http://marsrovers.jpl.nasa.gov/gallery/all/1/p/973/1P214568457EFF76POP2403L5M1.JPG
		Rover: Opportunity    Sol: 986    Instrument: Panoramic imager    Filename:1P215722628ESF76POP2416L5M1.JPG Link: http://marsrovers.jpl.nasa.gov/gallery/all/1/p/986/1P215722628ESF76POP2416L5M1.JPG

There are many more of these on the Martian surface.  I have identified at least 20 of them, and all fall into two basic shapes- some have points on both ends, some have a point and a blunt end.  In both cases they can be fitted into 5 or 6 pieces to form a spheroid.  The closest fossil creature to the structures (and an excellent match for the shapes) is a blastoid.

When these fossil break into pieces, the results are rind objects that are identical to those we find on Mars.  The first and last specimens are most like the fossils I have found in the Opportunity images.  My conclusion is that we are seeing blastoid fossils scattered all over the planet.  They have been imaged by both rovers and by all of the instruments (microscopic, panoramic, navigation, etc.)  Some specimens also faintly show sutures or "corrugations" on their edges.  in particular see the specimen from Sol 614.