14 ON THE CARDIAC VALVES IN ECHIDNA ETC. [Jan. 16, 



the muscular bands found on its deep septal face (noted above in my 

 description of it) are the incipient attachments of the ventricular 

 muscular wall described as characterizing the second stage of deve- 

 lopment of the mammalian heart (see above). Accordingly we 

 have no reason for regarding the minute rudiment of a septal flap 

 which occurs in some Ornithorhynchus-hearts as corresponding to 

 the septal flap of the Crocodile's right ventricle. There is no ground 

 for regarding that rudiment as a part of the primary endocardial 

 valve. It is rather an incipient and abnormal extension of the 

 secondary valve, the muscular trabecalae involved in the development 

 of the secondary valve being, in these cases, widely connected so as to 

 encroach on the septal face of the ventricle in place of being restricted 

 completely to the anterior face. Between this condition and that of 

 the fully formed septal flap of other mammals there is a wide gap. 



There seem to be no facts which would lead one to trace the 

 Monotreme-heart to an ancestor in which the secondary muscular 

 valve was fully developed both in its anterior and its septal portions. 

 From such an ancestor it would, no doubt, he possible to derive the 

 Monotremes, on the one hand, by suppression of the septal portion, 

 and the other Mammalia, on the other hand, by retention of the 

 whole valve with degeneration of its muscular substance. But in 

 view of the fact that the secondary muscular valve is not found to 

 develop a septal portion in any other Vertebrates than the higher 

 Mammalia, it is more probable that the ancestor of the Monotremes 

 had no such septal development, that the rudiment of such septal 

 development found in rare cases in Monotreme-hearts is an incipient 

 rudiment, and that the full development of the septal half of the 

 secondary right cardiac valve (in Bernays's sense) is a new departure 

 and special characteristic of the non-cloacal Mammaha. 



EXPLANATION OP PLATES III. and IV. 



Letters in botli Plates, 

 ft. Great anterior muscular column or papillary muscle. 

 b. Eight anterior muscular column. 

 b'. Second (extreme) right anterior muscular column. 

 e. Left anterior or " conal " muscular column. 

 cc, y. Columnaj carnesB connecting the base of the great anterior muscular 



column with the anterior ventricular -wall. 

 rac. Eight anterior membranous cusp. 

 lac. Left anterior membranous cusp. 

 xc. Downward vertical extension and attachment of the membrane of the valve 



(possibly to be regarded as a rudiment of a septal flap). 

 pf. Membranous septal flap of the right cardiac valve of the Crocodile. 

 PA. Base of pulmonary artery, 

 i? V. Eight ventricle. 

 L V. Left ventricle. 



Plate III. 



Figs. 1, 2, 3. Three dissections of the right ventricle of a heart of Omitho- 

 rhynchus. 

 4. Similar dissection to fig. 2, of another heart of Omithorhynchus. 

 .5. Eight ventricle of the Cassowary. 

 6. The same with the cardiac valve divided. 



