Hi Mark,
There are a number of different types of shark teeth that have bilateral
cusps beside the main blade.  But given the large size of your specimen, it
is probably an Otodus tooth, probably Otodus obliquus (a larger ancestor of
the modern day mackerel shark).

Here are some site that may be helpful:

http://nmnhwww.si.edu/paleo/sharkteeth/#anchor26517478
http://www.megalodonteeth.com/
http://www.naturepreserved.com/megalodon/fossil%20shark%20teeth.htm

        Regards,
        Paul M.