crustose lichenCrustose lichens look like... crust. The pale splotch at the bottom of the picture at the right shows crustose lichen on the smooth bark of a tree. The picture shows things about twice the size they really are.

In the picture, notice that at the top and a little to the right there's a much smaller blob of similar pale color, but this blob's edges are free. That's because the top blob is a foliose lichen. Notice how the big crustose lichen at the bottom has indistinct edges that don't turn up at all. This is one way to distinguish crustose from foliose lichens.

Sometimes it's hard to tell the two apart because young colonies of foliose lichen can have poorly developed edges and look very much as if they were crustose. You need to check several colonies before making a decision. Notice that in the crustose-lichen's center there are some small curdle-like bumps. Later these will develop into tiny cups -- the lichen's spore-producing reproductive structures.