1882. ] OF ORNITHORUYNCHUS PARADOXUS. 555 
papillaris, although nut  papillary”’ (figs. 12 & 13,@). 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 chordee tendineze 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 chord tendinez 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 (4) 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 chordee tendinez 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 fap.—To the right or posterior border of this smaller mus- 
cular band 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, pc). 
The condition seen in heart No.1 may be understood by sup- 
posing, in a valve arranged as in Man, the chordz tendinez to be- 
come muscular and compacted together and so to form parts of the 
