We thank D. Jones, R. Portell and E. Vokes for collection of and access to many of the specimens. L. Anderson, S. Bursten, K. Driver, K. Flessa, L. Leighton and G. Vermeij provided helpful comments and discussion, and P. Kelley and an anonymous reviewer provided helpful comments on the manuscript. We are greatly indebted to L. Schuff for assistance in data collection. This document was prepared with LATEX and the tth (TEX to HTML) converter (I. Hutchinson).