thanks Richard, hadnt given any consideration to that being the reason for different shapes.
Are the petal ones not prone to letting in light a bit more than conventional hoods.
Also I notice on my lens hood one of the pairs of petal shape are larger than the other pair, which should be at the top of the lens ?
thanks
Andrew
No, a petal hood is more efficient than a round one, that could only be as deep as the shallowest part of the petal, if you see what I mean. The petal design allows the hood to be made deeper further away from the corners. You'll notice that the petals are different heights along the sides compared to top and bottom.
Ideally the petals should not be squared off at the end, but they often are so that you can put the lens down and it won't fall over.
In terms of efficiency though, lens hoods for zooms are not very good, because they can only be at the maximum depth at the shortest end of the zoom, or they'd protrude into the image. Also, hoods designed for full frame lenses are not as deep as they could be for use on crop-format cameras (with their narrower field of view). Sigma makes hood extensions for some of its lenses for this reason.
The perfect hood should be rectangular to match the image, and as deep as it can be without intruding into the picture. It should also 'zoom' with the lens, like this one I made earlier

It makes a very noticeable difference in really difficult light, eg sun just outside the frame, or for studio work where there's a lot of bright white background outside the image area, which is what I use it for.
Standard hood
Hood set at 80mm
Hood set at 160mm