vi HEAET OF LEPIDOSIEEN 379 



forms the common foundation of auricular and ventricular septum, 

 traverses the auricular canal, projecting into it from behind so as 

 to give the atrioventricular opening a horse-shoe shape. It is the 

 part which lies above (dorsal to) the opening which becomes 

 thickened and eventually assumes a cartilaginous character to form 

 the plug. The plug is to be regarded as the homologue of the 

 posterior atrioventricular cushion of Elasmobranchs but Eobertson 

 failed to find any trace of an anterior cusbion. 



The conus in the completely developed state is characterized by 

 the absence of the functional valvular apparatus found in its 

 homologue in other Vertebrates. On the one hand the endocardiac 

 ridges, functional in the young Elasmobranch or Ganoid, are no 



■ Fig. 180. — Heart of an adult Lepidosiren with the right side removed. 

 (After J. Robertson, 1913.) 



AV.p, atrioventricular plug ; cy, coronary vein (cut) ; d.C, ducts of Cuvier ; p.v, pulmonary vein ; 

 p.V.c, posterior vena cava at its opening into the sinus venosus ; s.A, atrial septum ; s.K, ventricular 

 septum ; s.v, sinus venosus (its opening into the right auricle indicated by an arrow) ; sp, spiral valve ; 

 III, VI, aortic arches cut near their ventral ends. 



longer in a condition to fulfil their original function and on the 

 other the pocket valves are here vestigial. 



Musculaeization of THE Heart. — Bud -like projections from 

 the myocardium grow into the cavity of the heart, meet together 

 and in the ventricle form a muscular spongework of the same type 

 as that seen in Elasmobranchs. In the case of the ventricle numerous 

 trabeculae arising in this way converge upon the free edge of the 

 atrioventricular ridge and become continuous with it. As develop- 

 ment goes on this spongy mass of trabeculae undergoes condensation 

 and acquires the solid character of the fully developed septum. The 

 septum is continuous dorsally with the atrioventricular plug and it 

 forms a muscular apparatus by which the plug is pulled down so as 

 to fit into and close the opening. The myocardium of the auricular 



