1894.] 



VISCERAL AKATOHY OF ORNITHORIIYNCHTTS. 



721 



" he" But between the apparent end of this flap and the muscle 

 " e " was a slight thickening, ligamentous in texture, of the inter- 

 ventricular wall of the heart. I hardly noticed this until I had 

 examined the second heart. In the latter the septal flap of the 



Fig. 3. 



r~Sp 



Heart of Ornithorhynchus, with right ventricle opened. 

 up, septal flap of valve. a, a', severed muscles of outer valve-flap. 



right auriculo-ventricular valve ivas completely developed. It was 

 tied down to the interventricular septum by two slight papillary 

 muscles, which, however, did not, as in the case of the other half 

 of the valve, at all invade the tendinous tissue of the valve. They 

 were attached to its edge merely by the tendons. This septal 

 half of the valve lay close to the interventricular wall, as, indeed, is 

 generally the case among mammals ; in the first of the two hearts 

 it was so little detached from the veutricular wall as almost to 

 suggest a case of cardiac disease rather than a normal structure. 

 But the state of affairs in the second heart showed plainly how it 

 was necessary to interpret the first heart. It will be noticed that 

 my observations agree entirely with those of Gegenbaur, particu- 

 larly in the case of the first heart, in which (to quote from Prof. 

 Lankester's translation of Gegenbaur's paper) " no trabecular pass 

 Proc. Zool. Soc— 1894, No. XLVIII. 48 



