Short version for subject line. Expanded it would be "I saw a triceratopsian shield* that had grooves in it for veins. I'm wondering how the bone shaped itself to provide that channel?" The groove would provide protection for the veins on three sides, and if deep enough the vein wouldn't protrude above the general level of the skin on the shield. *Rather like an elephant ear with a bone plate in it. Just wondering. I'm sure Jack Horner could explain it, but I don't like him much.