Dear Jocelyn, There is a very nice, inexpensive, key to mammal skulls and lower jaws available for about $10.00. The title is, "A Key-Guide to Mammal Skulls and Lower Jaws" by Aryan I. Roest (1986). The guide was published by Mad River Press, but I found it for sale online using Bookfinder.com which showed the lowest price of $10.00 currently at Amazon.com
Yours, Joseph Cornell Idaho State University
