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Show 1882.] OF ORNITHORHYNCHUS PARADOXUS. 557 The "mitral " valve of the Ornithorhynchus is in fact a "tricuspid" instead of a " bicuspid " valve. Its construction, so far as the relation of membrane to muscle is concerned, is similar to what is seen in Man's heart. There is no invasion of the membranous flaps by the fleshy substance of tbe musculi papillares as on the right side of the heart; at the same time the connexion of the musculi papillares with the membrane of the valvular flaps is direct, and not by the intervention of chordae tendineae. In Man's mitral valve there are really four groups of chordae which pass from the membrane to the heart's wall or to musculi papillares. A broad flap of membrane is developed between the anterior pair of these groups of chordae, and, again, between the posterior pair, but not between adjacent anterior and posterior groups. In Ornithorhynchus the attachment of the membrane to the muscle is by three equidistant points of the valvular membranous collar to three elevations of the muscular substance of the ventricle ; and, as shown in the figure (fig. 17), the membrane is equally developed in each of the three spaces between the attachments. It is thus divisible into three areae, each having the form of a truncated triangle. The valve is indeed more nearly comparable in shape to the aortic trisegmented semilunar valves than to the mitral of the human anatomists. A very distinct and important point of resemblance between the left auriculo-ventricular valve of Ornithorhynchus and the semilunar valves at the base of the great arteries, is the existence of a small knob of cartilaginous consistence at the centre of the free margin of each triangular portion of the valve. These appear to have the same significance as the corpuscles of Arantius in the semilunar valves. THE AURICLES OF ORNITHORHYNCHUS. Meckel has remarked on the large size of the right auricle of Ornithorhynchus as compared with that of the left. He has also stated that there is a very deep fossa ovalis. In these statements Owen is in accord with him. Gegenbaur does not discuss this subject when treating of the right auriculo-ventricular valve. I find that the right auricle is of unusually large proportions in Ornithorhynchus (figs. 5 & 6), and have compared in the drawings given the proportions in this animal with those presented by the Rabbit. In fact the right auricle is much larger than has been hitherto supposed ; for what Meckel and Owen have taken for a fossa ovalis appears not to be the representative of that structure, but an independent and special caecum of the right auricle by which it .encroaches upon the area occupied in other animals by the left auricle. The orifice of this caecum, seen on opening the anterior wall of the ri°-ht auricle, is very sharply defined and of the size which the fossa ovalis might be expected to present (Plate X X X I X . fig. 8, Ca). It is not, however, in the position proper to the fossa ovalis. It leads into an extensive sac; and at first I was under the impression that the sac in question was a part of the left auricle, and hence that we had here a permanent communication between the |