The sedimentary rock formed from the compaction and cementation of skeletons and shells of sea organisms is limestone.

Here's why:

* Composition: Limestone is primarily composed of calcium carbonate (CaCO3), the same material that makes up the shells and skeletons of many marine organisms like coral, mollusks, and foraminifera.

* Formation: Over time, the accumulation of these organic remains at the bottom of the ocean gets buried under sediment. The pressure of the overlying layers compacts the material, and dissolved minerals in the water cement the fragments together, forming solid limestone.

While there are other types of limestone, the formation you described is the most common and a key example of how sedimentary rocks form from biological processes.