Attached Link: http://www.biosbcc.net/ocean/marinesci/05nekton/mthist.htm

Just Google/Images on tortoise shell. The first page of results has full shells and sliced sections. Once you see how the pattern and colors are arranged, you can make your own. This site has a nice photo of the texture: http://www.biosbcc.net/ocean/marinesci/05nekton/mthist.htm