This research is part of my Ph.D. dissertation. I gratefully thank my dissertation director, E.H. Lindsay for his guidance through out this project. I also thank the late J.A. White and the late C.A. Repenning for their help with lagomorphs and shrews, and Y. Petryszyn for convenient assistance when I examined the specimens at the University of Arizona Mammal Collection. R.J. Zakrzewski provided a review on an early version of the paper. I have received great help in the field from the late W.R. Downs, K.F. Downing, L.E. Park, L.J. Roe, O. Fejfar, J.L. Pederson, C. Pederson, K. Iris, J.H. Harrison, E.E. Erickson, and C. Lindsay. I would like to give my special thanks to C.A. Repenning, J.A.White, and W.R. Downs, who had contributed their lives to vertebrate paleontology.
This research was supported by the NSF grant EAR 9526386.