Sounds like maybe the Clavate Tortoise Beetle (Deloyala clavata)
It the 2nd one on the list he
http://www.ladybuglady.com/goldenbeetle.html

"donald girod" wrote in message
...
I have this ugly little thing eating holes in my potato leaves. I can't
find it in my insect book, but it should be called the melanoma bug. It
is
almost circular, 5/16" in diameter, very flat, with an ugly black wart on
its back that looks like the pictures at the dermatologist. The fringe
around the wart is almost transparent, so that it looks like a black wart
on
the leaf.

What is it? It must be common enough but as I say, my Peterson's doesn't
seem to mention it.