A field mouse.
The Harvard Crimson continues its relationship with Cambridge's Inspectional Services. After exploring the sanitation of local Harvard Square restaurants, our intrepid reporters turn their eyes toward rodent problems in the Houses. The article contains a ridiculous quote:

A mouse's body is all muscle except for one tiny bone in its head, says Packer, who has been inspecting in Cambridge for the last 12 years.

�This is how big of a space they need,� he says holding up a ball-point pen horizontally and pointing at the tiny circle top of the cap.

While I am not sure how small of a hole a mouse needs to enter a building, I am pretty sure that mice have full skeletons.

The above image of a field mouse is cut from a photograph by Aaron Logan. Since his work is licensed under a Creative Commons Attribution 2.5 License, the above image is similar licensed.