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 rnuscuh papillares 

 with the membrane of the valvular flaps is direct, and not by the in- 

 tervention of chordae tendiueae. 



In Man's mitral valve there are really four groups of chortise 

 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 chordae, 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 arese, each having the form of a truncated 

 triangle. The valve is indeed more nearly comparable in sha})e to 

 the aortic trisegmented 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. 



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 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 cfficum 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, 

 Cce). 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 left auricle, and 

 hence that we had here a permanent communication between the 



