Re: How would you identify and unknown animal bone?

Size matters, Talitha !

Once you have decided that the bone belongs to a land mammal, the length 
should give you a fairly accurate guide to the size range of the animal.  
Even in a continent with as diverse a fauna as N. America (did it come 
from there ?) there are relatively few animals to choose from within each 
size range.

Having narrowed down your choice on the basis of size, the next thing to 
do is assign it to a major group, probably at the order level.  Is it a 
carnivore, an artiodactyl, a lagomorph, etc ?  One clue would be to look 
for signs of the fibula.  In some mammal groups the fibula is fused to the 
tibia, in others it is free but may have distinctive articulations at one 
or both ends.  

It is the shape of the articulating surfaces at each end of a limb bone 
which give the most reliable clues to identification.

The only reliable way to discover the nature of these articulations, and 
then to move on to your final identification, is with reference material.  
The differences in shape between limb bones are often so subtle that even 
the best drawings and photos are of limited use, and you need to be able 
to hold your bone side by side with a known example for a conclusive 
identification.  There is also the added problem that very few such books 
exist, and they are all out of print, as far as I know !  Professional 
conservationists and zooarchaeologists always have a reference collection 
available for critical identifications. 

Don't give up just because you haven't got access to a top-class reference 
collection !  (You might have, if you ask at your local natural history 
museum or university zoology/archaeology department).  If you can persuade 
your parents to have a roast leg of lamb next weekend - there's your 
artiodactyl reference specimen, and your school may have a dog or cat 
skeleton set up in the lab - there's your carnivore.

Good luck - and remember that being able to explain the reasoning behind 
each step of your identification is as important as arriving at a final 
species name, which is not always possible.