In an interesting recent paper, Kapoor et al. 2003* Show the Circle of Willis in several different species. Their key is either absent or well hidden. This may help!
NB., If the authors read this and feel I am in error. Sorry! Please just email me and I'll correct it!
Anatomia, Histologia, Embryologia
Volume 32 Issue 6 Page 347 - December 2003
Morphology and Comparative Anatomy of Circulus Arteriosus Cerebri in Mammals K. Kapoor, V. K. Kak and B. Singh