<p><i>Peltigera horizontalis</i>
20090514.15
Wells Gray Provincial Park, BC
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Also called "flat-fruited pelt".  I like "concentric" because of the (ocasionally) striking arrangement of the rhizines in concentric circles near the edge on the undersurface.  I'm told you can tell the age of the lichen based on these "growth rings".
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/7147684@N03">Jason Hollinger</a>
CC BY 2.0
<p><a rel="nofollow" class="external text" href="https://www.flickr.com/photos/7147684@N03/3536828106/">Concentric Pelt</a>
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<ul><li>Uploaded by <a href="//commons.wikimedia.org/wiki/User:Amada44" title="User:Amada44">Amada44</a></li></ul>