Since no sutures can be seen in this structure, it probably is the ventral, ossified portion of the basioccipital.
"A New Order of Fishlike Amphibia From the Pennsylvanian of Kansas" by Theodore H. Eaton
The median ridge on the basioccipital bone increases in prominence in older voles.
"A Population Study of the Prairie Vole (Microtus ochrogaster) in Northeastern Kansas" by Edwin P. Martin
In front of the basioccipital is the basisphenoid with an alisphenoid on each side.
"Encyclopaedia Britannica, 11th Edition, Volume 14, Slice 3" by Various