Iowa State University Veterinarian Records, RS 22/6/0/9, University Archives, Special Collections Department, Iowa State University, http://www.add.lib.iastate.edu/spcl/arch/rgrp/22-6-0-9.html
In the same manner as in other domestic animals, the age of dogs can be ascertained mainly on the basis of an examination of the teeth. Ordinarily, the adult dog has forty-two teeth: twenty in the upper jaw and twenty in the lower, which are designated as incisors, canines, premolars and molars.
"Age Determination in Dogs,"
Iowa State University Veterinarian: Vol. 2
, Article 6.
Available at: http://lib.dr.iastate.edu/iowastate_veterinarian/vol2/iss1/6