PAUL HANCOCK and BRIAN J. SKINNER. "ammonites." The Oxford Companion to the Earth. Oxford University Press. 2000. Encyclopedia.com. 9 Dec. 2009 <http://www.encyclopedia.com>.
PAUL HANCOCK and BRIAN J. SKINNER. "ammonites." The Oxford Companion to the Earth. Oxford University Press. 2000. Encyclopedia.com. (December 9, 2009). http://www.encyclopedia.com/doc/1O112-ammonites.html
PAUL HANCOCK and BRIAN J. SKINNER. "ammonites." The Oxford Companion to the Earth. Oxford University Press. 2000. Retrieved December 09, 2009 from Encyclopedia.com: http://www.encyclopedia.com/doc/1O112-ammonites.html