They are rounded in shape and allow the driver to see the front of the bus and the space in front of it.
This allows a driver to know whether or not there is a child in that dangerous blind spot.
The other morning I was driving an early morning country route, and the horizon was just beginning to lighten directly behind me.
I happened to glance at that mirror and it was a very beautiful sight. The mirrors distort the view, and it was a very interesting effect.
The sky above the bus was darkest blue, getting lighter and lighter as you moved towards the bus. Then there was the bright yellow bus itself with all of its lights. It was quite something, and I'm afraid my description leaves much to be desired. I'll have to try to capture it with a camera at some point.