In response to cuttlegirl from another thread, here's a photo of the Devonian fossil ammonoid Tornoceras mesopleuron. Specimen is 46mm. I based the illustration that is my avatar on this specimen, a rare species found in Ontario. For info. on this genus see: House, M. R. 1965. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences. Vol. 250 No. 763 pp.79-130.