Reading this, I had to wonder what was "opposite" about these "opposite birds". Apparently it's how the shoulder blade (scapula) connects to the coracoid bone (a bone not present in therian mammals). From Wikipedia's article on Enantiornithes:
> Specifically, in the Enantiornithes, the scapula is concave and dish-shaped at this joint, and the coracoid is convex. In modern birds, the coracoscapular joint has a concave coracoid and convex scapula.
> Specifically, in the Enantiornithes, the scapula is concave and dish-shaped at this joint, and the coracoid is convex. In modern birds, the coracoscapular joint has a concave coracoid and convex scapula.