Unfortunately, nearly impossible to tell what this is, from these photographs. The shape and cracked pileus may suggest a Calvatia, but without pulling it up and slicing it (to see if it is a puffball or has gills/pores) I can't be sure.
I agree with Christina's suggestion of Calvatia. The surface from the top down to the 'stem' seems too solid and uniform to fit an agaric with a universal veil.