There is a great demand for statist ical modeling of phenomena tha t evolve in bo th space and time. Practical examples are those in Haslett and Raf tery (1989), Handcock and Wallis (1994), Cressie and Huang (1999), Brix and Diggle (2001), Stroud et al. (2001), De Iaco et al. (2002), Gneit ing (2002), and Hartfield and Gunst (2003), to mention but a few. Two commonly used tools to describe the ...