518 



ANATOMY OF VERTEBRATES. 



the placental Mammalia, relates to the very brief period during 

 which the auricles intercommunicate in the Marsupials, and to 

 the minute size, and in other respects incompletely developed 

 state, at which the young marsupial animal respires air by the 

 lungs, and has the mature condition of the pulmonary circulation 

 established. The right and left auricles intercommunicate by an 

 oblique fissure in the uterine embryo of the Kangaroo when 

 two-thirds of the period of gestation is past, but every trace of 



401 



402 



Heart of the Kangaroo. 



Heart of the Wombat. 



this foetal structure is obliterated in the subsequent growth of the 

 heart ; so that in the mature animal the wide terminal orifice of 

 the postcaval, ib. d, is separated from that of the right precaval, 

 ib. b, by a simple crescentic ridge, ib. e, which forms a salient angle 

 of the parietes of the auricle between these apertures. The orifice 

 of the left precaval, ib. c, is close to that of the postcaval, in a 

 position analogous to that of the coronary vein in Man, which 

 here opens into the left precaval. The right auriculo-ventricular 

 valve is membranous, and its free margin is attached by fine 

 ( chordae tendineae' to three mammillary ( columnae carneas ;' these 



