30 correlative economy. Thus, as Gegenbaur (Vergleichende Anatomie, 2nd edition, p. 836) has remarked, the intervalvular space in these animals corresponds to the entire cavity of the spongy walled heart of fishes and amphibia; and the sinuous intertra- becular cavities in the spongy walls of these latter animals correspond with the chief part — viz. the extravalvular part of the ventricular space — in mammals, birds, and Crocodilina. Now,. the musculi papillares represent the disposal or destination of the innermost layer of the right ventricle, ac- cording to Dr. Pettigrew (see his paper, Phil. Trans. 1864, p. 479); and I would submit that the moderator band is but a specialisa- tion of the next layer in order from within outwards — to wit, Dr. Pettigrew's sixth layer, which he has figured (Plate XIV. fig- 33) as proceeding in a spiral direction from right to left, much as the fibres of the moderator bands I have figured do. A study of the heart of the rabbit will put this matter in a very clear light, and further open our eyes to see and recog- nise the rudimentary representation of this
