Hey Eleanor. I had a very difficult time locating any images of the giant beaver ribs, however at
Casting the Earlham Castoroides, Part 2
There are several images of the ribs, and although not close up, seem to be similar and I thought that my find was possibly about the correct size. I have previously found a Castoroides molar at this particular beach. A lot of it was a guess, and would greatly appreciate assistance. Thanks…Lance