556 PROF. E. R. LANKESTER ON THE HEART [June 20, 
papillary muscles, which should then be continued right through 
the membranous collar of the valve to the auriculo-ventricular ring. 
Further, we should have to suppose the suppression of the whole of 
the septal division of the valve and its related chordze and muscles, 
excepting a little piece in immediate relation with the right anterior 
papillary muscle. 
The heart No. 2 (fig. 13) of Ornithorhynchus differs from No. 1 
in the fact that there is not a single right anterior papillary muscle, 
but five separate muscular slips representing it, of which three (fig. 13, 
b, 6, b) are attached to and traverse the membranous substance of 
the valve, whilst two (m, 7) pass over it and reach the auriculo-ven- 
tricular ring. These latter are of great importance in the comparison 
with the heart of Sauropsida, since they arise from the anterior 
ventricular wall. The heart No. 2 is also remarkable, as already 
mentioned, for the reduction of the septal portion of the membranous 
valve to a vanishing quantity (fig. 16 pc). 
In both hearts the three muscles of the valve (or two and the 
subdivided third) become continuous with one another at their inser- 
tion into the auriculo-ventricular ring. The large papillary muscle 
in the middle spreads out on either side, and on the left completes the 
arch formed by the muscular lobe (e) rising from the side of the pul- 
monary cone; on the right it similarly completes an arch, of which 
the right side is formed by the spreading insertion of the right papil- 
lary muscle or its representative slips (figs. 12, 13). 
It is thus quite clear that the description of the right auriculo- 
ventricular valve of Ornithorhynchus as “fleshy ”’ is quite correct ; 
membrane, though present, plays a subordinate part as compared 
with what is seen in other Mammalia. 
Not only this, but it seems probable, from the very imperfect 
development of a septal or posterior flap to the valve, that the action 
of the valve must differ importantly from that of the valve of other 
mammals, and resemble that of the fleshy valve of birds and reptiles. 
The reflux of blood into the auricle on contraction of the ventricle 
is not prevented in Ornithorhynchus by a passive floating-out of 
membranous cusps, but, as in birds and reptiles, the muscular arches 
of the valve are by their active contraction pressed against the septal 
portion of the auriculo-ventricular orifice, which is for two thirds (or 
sometimes more) of its extent devoid of any depending cusp or mem- 
branous flap. 
How far there is a real agreement, due to common inheritance 
from a common ancestor, between the muscular structures of the 
right cardiac valve of Ornithorhynchus and those of the similarly 
placed valve in birds and certain reptiles, is a distinct and very 
important question, into the discussion of which I do not propose 
to enter on the present occasion. 
Tue Lerr AURICULO-VENTRICULAR VALVE. 
The valve which is called “ mitral” in Man’s heart differs in struc- 
ture in Ornithorhynchus from the human standard, although much 
less so than does the right auriculo-ventricular valve. 
