canon lens to mamiya body ..... not that i know of and not likely to have one ..... why? lenses like canon's (as an example) FD and EF lenses are designed to maximize capture of an image onto a sensor/film up to 24x36mm .......... not a 60x45mm or larger film/sensor size .... there will be a huge vignette
you can mount a medium format lenses to a smaller format cameras, but not the other way round, as the image circle of these lenses won't provide enough coverage, and also not possible to focus at infinity on a medium format cameras.
ed9119 and uncle catchlights have given great advice on the issue of image circle coverage. Apart from image circle Canon lenses have a much smaller flange focusing distance meaning u cannot achieve infinity focus
While there are adapters adapting the manual focusing M645 lenses to Canon bodies, I am not sure if anyone makes adapters for newer AF Mamiya 645 lenses ( U r using them i suppose, on ur 645AFD II ? ) to Canon bodies ( or other smaller format lenses for that matter ). If i am not wrong, the AF Mamiya lenses got no aperture ring to speak of , u need a pretty complicated adapter to control aperture in that instance ..
* i will be pretty interested if there is an adapter that allows the newer AF Mamiya lenses to be adapted on EF mount too
This link let you know why EF mount cannot be used on mamiya 645. Your adapter need to have glass to allow the EF lens to focus at infinity. Without added glass elements, you get only a macro effect, similar to using extension tubes on your lens.