Here's another fossil urchin I found yesterday. It is embedded in a flint matrix - I chipped off a corner and the flint is continuous through the fossil.
I took the picture above to see if I could get a good shot of the ocelli - three secondary eyes between the big ones. They are just about visible. Why spanning tree? It's a routing term, from each user's perspective the network looks like a spanning tree - with no loops and only one route to any place. Every user's spanning tree is unique.