Hey, C'nor, I don't know much about mouse skeletons, but I do know that humanity and primates are unusual in only having one lower jaw bone. I have two mouse skulls in my collection, taken from owl pellets, but neither came with a lower jaw. My shrew skull, though, did, and it's split. But shrews and mice aren't closely related, so that's actually irrelevant rambling. As was that sentence, and on closer inspection this one. Anyway, I think mice have two lower jaw bones, a left one and a right one.