34 band, which we sometimes see in the human heart, through the typical, and I should an- ticipate, constant, but not functionally im- portant, representation of it in the rabbit, up to the important and structurally promi- nent development attained to by it in the Ungulate mammal, and this solitary instance for the class of Birds, and the sub-class with such generalised affinities, of Struthiones. And, speaking of the method of gradations, I take this opportunity of saying that its application in the case of the muscular right auriculo-ventricular valve of birds will, in my judgment, put an end to the disputes which have taken place as to its homology with one or other of the two valves in the crocodiles. The two portions of the valve in the Casuarius australis are so nearly equal — the larger being 17 inch, as against 1 4 of the smaller — as to do away with the difficulty which might be felt in holding that both Crocodilian valves are represented here. There are other reasons for this view, which I reserve for another occasion. But whilst speaking of the heart of the Bird, I cannot forbear pointing out how the structural ar-
