sphenoid bone

It is a sphenoid bone; single, irregular bat(a bird)-shape bone forming part of the CRANIAL Floor. Not many people think sphenoid bone being part of the cranial bone that is why they don't find any answer to this question. but Sphenoid bone is one of the cranial bone and can be found on the cranial floor when looking at the skull from its superior view.

it is the bone that you can touch behind the last molar and it is apart of the sphenoid bone better known as the ramus of the sphenoid.

Posterior means behind or toward the back -- so which bone is behind the ethmoid bone? The sphenoid bone (butterfly shaped one).
The bone that is found just posterior to the ethmoid bone in the orbit is the sphenoid. Major paranasal sinuses are the ethmoid, sphenoid, frontal, and maxillary sinuses.

The maxillae contain the maxillary sinuses, the ethmoid and sphenoid contain the ethmoid and sphenoid sinuses, respectively.

****Maxilla is a facial bone, the three cranial bones that contain sinuses are ethmoid, sphenoid and frontal.