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

"The prominent bony ridge that you can palpate across the posterior surface of the scapula is the scapular spine".

Explanation:

A scapula is a plane and triangular bone located in the superior and posterior part of the thorax. The posterior face of the bone is divided into the supra-spiny fossa and infra-spiny fossa. These two areas are divided by the scapular spine. This is a bony formation that goes from medial to the lateral side of the scapula and finishes in one of the extremes as a bulky prominence called acromion.

The spine is easily palpable through the skin.