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 the 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 in- 
tervention of chorde tendinee. 
In Man’s mitral valve there are really four groups of chords 
which pass from the membrane to the heart’s wall or to musculi 
papillares. A broad flap of membrane is developed between the an- 
terior pair of these groups of chordze, and, again, between the pos- 
terior 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 areze, each having the form of a truncated 
triangle. The valve is indeed more nearly comparable in shape to 
the aortic trisegmeuted semilunar valves than to the mitral of the 
human anatomists. A very distinct and important point of resem- 
blance 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. 
Tue 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 sub- 
ject when treating of the right auriculo-ventricular valve. I find 
that the right auricle is of unusually large proportions in Ornitho- 
rhynchus (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 in- 
dependent and special cecum of the right auricle by which it en- 
croaches upon the area occupied in other animals by the left auricle. 
The orifice of this caecum, seen on opening the anterior wall of the 
right auricle, is very sharply defined and of the size which the 
fossa ovalis might be expected to present (Plate XXXIX. fig. 8, 
Ce). 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 im- 
pression that the sac in question was a part of the /eft auricle, and 
hence that we had here a permanent communication between the 
