18ti2.] OF ORNIXaORUYNCHUS PARADOXUS. 551 



apex directed backwards) in a nearly horizontal direction in the 

 valve. They occupy, however, scarcely the third part of the entire 

 lengtli of this division of the valve. At the broadest part of this 

 same division a muscular band passes from the ventricular septum, 

 and is inserted into the former, and spreads its fibres in a fan-like 

 expanse in the valve nearly up to the origin of the valve from the 

 margin of the ostium. A second smaller bundle lies behind this, 

 also arising from the septum. If we spread out the valve, and 

 compare the purely niembrauous surface with that provided with 

 muscular tissuf, the former is found to be larger than the latter. 

 The second portion of the valve arises from the part of the ostium 

 belonging to the septum. At its narrower part it is in continuity 

 with tlie other division of the valve, broadens out from behind 

 forwards, and is fastened to the septum along a perpendicular line 

 stretching from the ostium into the ventricle. It is therefore not 

 only fastened to the circumference of the ostium, but, starting fr9m 

 there, also to the septum. Since the latter Une of fixation is per- 

 pendicular to the line of origin along the ostium, this portion of 

 the valve forms a ' pocket-valve ' — the more so since no trabeculae 

 pass to its free margin, and moreover no muscular fibres can be 

 detected in its substance. 



" When a comparison of this arrangement is made with that of 

 Birds, the difficulty is at once obvious that in Ornithorhijnchus the 

 septal portion of the ostium has a valve, whilst such is wanting in 

 Birds. The whole apparatus cannot, therefore, be compared with 

 that of Birds, but only the portion of the valve which arises from 

 the outer half of the circumference of the ostium." 



In the absence of figures it is not possible fully to comprehend 

 Professor Gegenbaur's description ; but it seems to me probable that 

 the heart examined by him differed individually from those studied 

 by Meckel and Cuvier, and from the two examined by me. In these 

 two, as will be seen below, considerable differences were observed on 

 comparison one with another. 



The main point on which Gegenbaur insists, is the existence of 

 a septal portion to the valve ; it is on this account that he objects 

 to a comparison with the Bird's valve. But this septal portion seems 

 to have been exceptionally large in the heart studied by him. In 

 both my specimens it was small, and left the larger part of the 

 septal margin of the ostium unprovided with any valvular fold. At 

 the same time it was larger in one specimen than in the other. The 

 existence of a greater or less portion of the valve along the septal side 

 of the ostium does not appear to invalidate the comparison of the 

 main bulk of the valvular structure with that of the Bird's heart, 

 though the closeness of the agreement is diminished by the fact 

 insisted on by Gegenbaur, viz. that the muscular bands of the valve 

 arise in Ornithorhynchus, as in the Crocodile, from the septal wall 

 of the ventricle, and not from the free outer wall as in the Birds. 



Recognizing, as all anatomists must do, the great interest attach- 

 ing to the observation that in Ornithorhynchus muscular tissue to a 

 large extent invades and replaces the membranous structure which 

 Proc. Zool. Soc— 1882, No. XXXVII. 37 



