12 PROF. E. R. LANKESTER ON THE CARDIAC [Jan. 16, 



from the ventricular wall. There is a great anterior muscle a, and 

 two right anterior muscles 6 h. Near the pulmonary artery (left angle) 

 there is a well-marked left anterior or " conal " muscle as in Ornitho- 

 rhynehus (see pi. xl. of former paper, e). 



Whilst these papillary muscles of Echidna differ from those of 

 Ornithorhynchus in not traversing the membranous valve so as to 

 reach the auriculo-ventricular ring, it would be wrong to suppose 

 that they are attached to the membranous valve by chordae ten- 

 dinesB as in Marsupials and Placentals. They are not so, but are 

 distinctly intermediate in the character of their attachment between 

 Ornithorhynclius and the other Mammalia. The great anterior 

 muscle is wedged into the membrane of the valve (see Plate IV. fig. 6). 

 The adjacent right anterior papillary muscle has two slips of the 

 membrane of th? valve reaching down to it, approaching in character 

 . true chordse tendinese, whilst the extreme right papillary muscle (h') 

 is wedged into the membrane, as is the great anterior column. The 

 connexion between the muscle, e, and the membrane is also direct. 



The form of the ventricles of the Echidna's heart is more pointed 

 towards the apex than in the Ornithorhyuchuc and thus more bird- 

 like. The septal wall of the right ventricle is even more convex than 

 in OniWiorhynclms, and shows more numerous muscular ridges 

 (columnae carneae.) 



Generalization as to Heart of Monotremata. — We are now in a 

 position to formulate as a distinctive character of the Monotremata 

 amongst Mammalia a peculiarity of the right cardiac valve. This is 

 not its muscular constitution, but the deficiency of a septal flap. 

 Less important is the absence of chordae tendineae from the valve in 

 both Echidna and Ornithorhynchus, and the close adhesion of the 

 muscular substance of the papillary muscles to the membrane of 

 the valve. In Ornithorhynchus, but not in Echidna, contrary to 

 what is observed in other mammals, the muscular tissue of the 

 papillary muscles invades in greater or less quantity the mem- 

 branous valve, and is continued as one or more varying nmscular 

 columns traversing the membrane, to be inserted directly into the 

 auriculo-ventricular tendinous ring. 



Considerations derived from the Facts of actual Development. — An 

 admirable memoir by Dr. A. C. Bernays, of St. Louis, Missouri, on 

 the "Developmental History of the Atrio-ventricular valves," is 

 publi-jlied in the second volume (1876) of Gegenbaur's ' Morpiio- 

 logisches Jahrbuch,' and has come to my knowledge since writing 

 the account of my observations given above. 



Dr. Bernays's memoir is a careful exposition of facts, brought to- 

 gether under the direction of Gegenbaur, with the view of giving a 

 detailed basis for the conclusion already formulated by that philo- 

 sophic anatomist, viz. that " the atrio-ventricular valves are, together 

 with the chordae tendinese, (actually iu individual development) 

 differentiatious of a part of tlie original muscular network forming 

 the wall of the ventricle." In accordance with this conclusion, 

 Gegenbaur has already, in his ' Elements of Comparative Anatomy,' 

 observed that the muscular right atrio-ventricular valve of Ornitho- 



