Dr. Kurt Benirschke
Comparative placentation is an important research area in its own right and as a background to animal models of human placentation. I will not be posting detailed information about the placenta of different mammals. That function is served by a web site maintained by Dr. Kurt Benirschke (above), a retired placental pathologist with strong links to San Diego Zoo. You will find the web site at this link.
The index page currently lists over 150 species and the site is especially strong on primates and artiodactyls (even-toed ungulates). Each entry lists the relevant literature and includes some older papers that may not be found on PubMed or by other search engines.
White-faced saki Wikimedia Commons.
For a few species the entries on this web site constitute the only information available. As an example, one family of Neotropical primates is represented by the white-faced saki (Pithecia pithecia). I know of no other information on placentation in this family (Pitheciidae).