1883.] VALVES IN ECHIDNA AND ORNITHORHYNCHUS. 9 



muscular column (a in fig. 12, pi. xl.) dividing the anterior mem- 

 branous flap ot the valve into right and left cusps, a single well- 

 marked right anterior column {b in fig. 12, pi. xl), from which was 

 given off posteriorly a small " septal flap " (pc in figs. 14, 1.5, pi. xl.) 

 representing the septal membranous flap of a typical mammalian 

 right cardiac valve in a very reduced state. 



Heart No. 2 differed in two chief features from heart No. 1. 

 First, the right anterior muscular column was broken up into three 

 small slips instead of being present in one piece (6 6 6 in fig. 13, pi. 

 xl.), and two pecuhar additional muscular slips (m, n) were developed 

 in the same region ; whilst, secondly, there was no trace of any thing 

 which could be properly called even a rudiment of the septal mem- 

 branous flap of the valve, although the point of junction of the right 

 border of the membranous part of the valve with the wall of the 

 heart was indicated as a bare representative of such septal extension. 



The seven hearts since examined present further variations. 



The heart lent to me by Dr. Pye Smith has a closely similar 

 arrangement of the muscular bands and membrane of the right cardiac 

 valve to that exhibited by my No. 1 . It differs in having a smaller 

 rudiment of the septal valve than my heart No. i. 



Of Professor Balfour's specimens four agree more or less closely 

 with heart No. 2 of my former paper. The most extreme of these 

 is represented in Plate III. fig. 4, where it is seen that not only is the 

 right anterior muscular column represented by separate muscular slips, 

 but these are very small. There is a curious downward growth of 

 the membranous substance of the valve in this region, and an adhesion 

 of the vertically extended membrane to the septal wall of the heart 

 (Plate III. fig. 4, xc), which is similar to the condition of the same 

 part in the Echidna-heart (Plate IV. figs. 6, 7, xc). The important 

 feature about this Ornithorhynchits-heart, beyond the absence of a 

 septal valve-flap, is the evanescence of the muscular substance of the 

 right anterior column. 



In most marked contrast to this are the two other hearts of Professor 

 Balfour's six specimens. In both these the region of the right 

 cardiac valve, corresponding to the right anterior muscular column 

 of my heart No. 1, is particularly fleshy. In Plate III. figs. 1, 2, 3, 

 the more remarkably developed of these two hearts is drawn. It will 

 be seen that the right extremity of the valve is not merely traversed 

 by a muscular column, but rt has additional muscular substance de- 

 veloped in it, so that it presents the aspect of a broad fleshy area 

 (Plate III. figs. 1, 2, 3, b, b) with no membrane on its outer or extreme 

 right border. The muscular substance, in fact, rises up in a broad 

 sheet from the wall of the ventricle and is inserted into the anterioi' 

 part of the auriculo-ventricular ring in a form which resembles 

 that of the fleshy Sauropsidaa right cardiac v;;lve more strikingly than 

 does that presented by any other of the specimens examined. 



The membrane in this lieart is reduced very considerably in pro- 

 portional area as compared with muscle : a little further development 

 of m.uscular substance would bring about the union of tiie right 

 lateral muscular mass, bb, with the great anterior muscular column, a. 



