Determination of the species of a skull
If you're serious about this (and it appears you are), you'll benefit by starting with a Comparative Osteology book or webiste that discusses all the various characteristics of skulls that give you clues to exactly what you're dealing with.

This page on Amazon has a variety of such books. Pages such as this one begin with the basics and references help you to pursue the subject further.

On your specimen, although the missing teeth would be very helpful, important identifying features include the very large percentage of the cranium given over to the masseters, the eye socket placement, the size, the jaw shape, etc.