HEART OF LYENCEFHALA. 517 



upper half are attached to the two membranous portions of the 

 tricuspid valve ; the margin of the membranous part of the valve 

 is attached to the fixed wall of the ventricle by two small chordae 

 tendineae ; and the structure of the valve thus offers an interesting 

 transitional state between that of the Mammal and that of the 

 Bird. The origin of the pulmonary artery is provided with the 

 three usual sigmoid valves. The left ventricle has very thick 

 parietes, which form the apex of the heart ; the mitral valve is 

 membranous ; the larger flap is attached to two strong columnar 

 carneae ; the smaller flap also receives tendons from some smaller 

 columnae. The left auricle, c, receives two pulmonary veins. 

 In the Echidna the free appendix of the right auricle is slightly 

 indented. The terminal orifice of the right precaval is protected 

 by a membranous semilunar valve, extending from its left side. 

 The musculi pectinati diverge from a strong fasciculus which 

 extends from the appendix to the orifice of the inferior cava ; 

 this fasciculus bounds the left side of a wide fossa ovalis, which 

 is imperforate. The postcaval is protected by a large membranous j 

 Eustachian valve ; the left precaval terminates by a distinct 

 aperture to the left of the preceding, and is also defended by a 

 process of the Eustachian valve. The inner surface of the right 

 ventricle is more irregular than in the Ornithorhynchus ; the 

 free wall is attached to the fixed one by several columnae carneae 

 and short chordae tendineae : the tricuspid valve is membranous, 

 and consists of one principal portion attached to the exterior 

 circumference, and a smaller portion closing the outer angle ; the 

 free margin of the valve is attached to the extremity of a large 

 fleshy column, arising by different roots from both the fixed and 

 the free walls of the ventricle ; a short fleshy column is attached 

 to the left extremity of the valve ; some chordae tendineae are 

 fixed to its right angle. 



The heart of Marsupials offers no peculiarity in its general 

 outward form. The apex is less obtuse in some species, as the 

 Phalanger and Wombat, than in others, as the Kangaroo. The 

 serous layer of the pericardium is reflected upon the large vessels 

 near to the heart. The fibrous layer of the pericardium adheres 

 to the sternum. The appendix of the right auricle is always 

 divided into two angular processes, a, a, figs. 401 and 402, one 

 in front and the other behind the trunk of the aorta, o. The 

 right auricle presents the following marsupial conditions: — There 

 is no trace of a ( fossa ovalis ' or an ' annulus ovalis,' * and the 

 absence of these structures, which are present in the heart of all 



1 xx. vol. ii. p. 52. 



