From Shaw’s 1807 lecture at the Royal Institution:
"If we rank this animal according to the Linnaean arrangement of quadrupeds, it must of necessity belong to the order Bruta, being destitute of teeth; but if we rank it according to its general habit or appearance, it might find a place among the Seals and other web-footed quadrupeds. The fact however is, that it may be questioned whether it really and properly belongs to the tribe of Mammalia or not; since no examination hitherto made, of such specimens as have been brought over, preserved in spirits, have exhibited the least appearance of teats for suckling the young…"
Platypuses are curious creatures when it comes to lactation - we now know that they do indeed lactate and provide milk for their young, but not from “teats”. The mammary glands under the skin along a short line on the female’s stomach secrete milk when stimulated, and this milk gathers on a tough, fibrous hair. The puggles can then consume it in a manner that looks much like a duck drinking water or eating crumbs in a lake. It’s a very odd sight.
Zoological Lectures Delivered at the Royal Institution in the Years 1806 and 1807 by George Shaw. Pub. 1809.