The prominent bony ridge that you can palpate across the posterior surface of the scapula is the

Spine.

Explanation:

The spine is the most prominent feature of the posterior scapula. It runs transversely across the scapula, dividing the surface into two.

The spine of the scapula is a prominent plate of bone, it crosses obliquely the medial four-fifths of the scapula at its upper part, and separates the supra - from the infraspinatous fossa.