1882.] OF ORNITHORUYMCHUS PARADOXUS. 555 



papillaris, although uoi "papillary" (figs.l2& 13,a). It obviously cor- 

 responds to the great or anterior papillary muscle of the human heart. 

 The vertex of the triangle formed by the membranous flap of the 

 valve is bisected by this great papillary muscle, which widens out as 

 it passes upwards, and is inserted into the anterior border of the 

 auriculo-ventricular ring, thus dividing the membranous triangular 

 flap into two distinct pieces, a right and a left. It is therefore per- 

 haps not quite correct to speak of these two segments united by the 

 muscular band as one anterior flap : the flap might perhaps be 

 regarded as composed of two membranous segments united by a 

 median muscular band whichis the prolonged anterior papillary muscle. 

 The membranous substance of the valve, however, is distinctly con- 

 tinued beneath or on the lumen side of the muscular band, thus 

 uniting the two halves, which at first sight seem to be separated by 

 the muscular substance. Obviously the right segment corresponds 

 to the right anterior cusp, and the left segment to the left anterior 

 cusp, of the human tricuspid valve. 



The peculiarity of this region in Ornithorhynchus consists in the 

 fact that there are no chordae tendinese connecting the membranous 

 segments to the great papillary muscle, while the muscle itself is 

 attached directly to the membranous flap, and is continued through 

 it up to the auriculo-ventricular ring, so as to invade (over a broad 

 band-like area) the membrane of the valve by muscular tissue. 



There is also a complete absence of chordae tendinese from the left 

 border of the left anterior flap, which is, in their absence, directly 

 attached to a fleshy arch which extends from the side of the pulmonary 

 cone up to the auriculo-ventricular ring, where it is confluent with the 

 expanded insertion of the great papillary muscle (figs. 12, 13, 14, 15, e). 

 Consequently the left membranous segment of the valve is triangular 

 in shape, the free border forming the base of an isosceles triangle. 

 Both hearts examined agreed in the features so far described. 

 In regard to the connexions of the right membranous segment of 

 the anterior portion of the valve, they differ a little from one another. 

 In heart No. 1 (fig. 12) there is a well marked right papillary 

 muscle {b) corresponding to the similarly placed muscle in Man, but 

 differing from that of Man, just as does the great papillary muscle, 

 in the fact that it is devoid of any chordae tendineae and is continued 

 from the ventricular wall up to the auriculo-ventricular ring, having 

 the right border of the right membranous cusp or segment attached 

 directly to its left border in the upper part of its course. 



Septal fiap. — To the right or posterior border of this smaller mus- 

 cular baud in heart No. 1, one lateral border of the very small septal 

 membranous flap is attached. This rudimentary posterior or septal 

 flap arises along the posterior or septal margin of the auriculo-ven- 

 tricular orifice for not more than one third of the extent of that mar- 

 gin, the rest being free from any valvular collar or fringe (figs. 14 & 

 15,;>c). 



The condition seen in heart No. 1 may be understood by sup- 

 posing, in a valve arranged as in Man, the chordae tendineae to be- 

 come muscular and compacted together and so to form parts of the 



