Lankester, E.R. 1877. Notes on the Embryology and Classification of the Animal Kingdom: comprising a Revision of Speculations relative to the Origin and Significance of the Germ-layers. Quart. J. Microsc. Sci. (N.S.) 17: 399-454 [4] S. 453
RICH, T. H. et al. Monotreme nature of the Australian Early Cretaceous mammal Teinolophos 2001 [2]
pnas.org
Rowe T., Rich T. H., et al (2008) The oldest platypus and its bearing on divergence timing of the platypus and echidna clades, Proceedings of the National Academy of Sciences 105 4, 1238-1242 [3]