The type of sedimentary rock formed by cemented shells of marine organisms is coquina.

Here's why:

* Coquina is a type of bioclastic sedimentary rock. This means it's formed primarily from fragments of once-living organisms.

* Shells are the most common component of coquina, and they are cemented together by calcium carbonate (often from the shells themselves).

While other sedimentary rocks like limestone can also be formed from marine organisms, coquina is specifically defined by its large, easily recognizable shell fragments.