Hericium, lion's mane

All three of the Hericium spp. share one common structure, teeth - which are easy to pinch off or cut through with even a dull blade. 

Often called Lion's mane, on account of it's mane-like shape, this saprobe (eats dead/decaying material) is easily distinguishable from the other Hericium spp. by it's ball or cluster-like shape. 

Rather than forming a ball, these two form branched structures from which the teeth hang. H. coralloides typically has teeth .5-1cm in length while H. americanum has teeth that hang a longer .5-4cm in length and sometimes looks shaggier. Both are edible.

 Photo by Cyndee Helms 

Hericium tend to develop as a tight fungal fruitbody (still easily sliced) with teeny teeth that will begin to elongate as the fruitbody matures. The tips of the teeth will begin to discolor to a more yellow-tan with age and may impart a slightly bitter flavor. 

Hericium are saprobes, consuming the dead or dying wood of hardwoods (though they aren't picky). Look up to find them.

One of the more common mushrooms that is sometimes confused for Hericum is Spongipellis pachyodon due to the elongated teeth. S. pachyodon is much tougher (think shoe leather) and the teeth are wide or oblong in shape. Furthermore S. pachyodon has a cap-like surface underneath-which the teeth hang. Hericium erinaceus can have a 'cap-like' structure in age, but there is a less defined boundary between cap and teeth.