How to get a pointer to the first element in a matrix

I am making a .NET wrapper to for the Newton Game Dynamics Engine. I would
like my wrapper to be able to use DirectX vectors and matrices. My problem
is that Newton takes an array of floats when it uses these. Therefore I need
to pass a pointer to the first element of the matrix (if it is a matrix I am
trying to pass). How do I get this pointer?

I could also copy all the elements into a float array each time but that
would be very inefficient.

Thanks in advance,
Per Rasmussen