Looks like you are well on your way to having yourself a pretty groovy billfold.. When I am doing skulls, the modeling tools....spoon and ball...get the workout as they are the tools that make the depressions and shasows (okay the small pear shader as well). The finish really sells the deal. I use a clear acrylic as a resist on the bones and then hi-lite stain over the rest with a bit of brush work with the darkest color. Are you planning on coloring your snake in some fashion?