CEHATODUS, PEOTOPTERUS, AND CHIM^EA. 603 



lung, the representative of which is absorbed in Chimoera, With these changed 

 conditions of other organs the heart and the brain have varied concomitantly, without 

 our being able to say exactly of what value to the organism the more amphibian brain 

 and the more amphibian heart of Protopterus, as compared with those of Chimcera, 

 may be. 



In figure 13 is given a surface-view of the heart of Chimcera monstrosa, as seen from 

 the front, the heart being freed from pericardium. The heart has the usual form and 

 proportions of that organ in Selachians — an obtuse broad ventricle free at the apex, 

 voluminous auricle, and a straight cylindrical cone. 



The ventricle and cone were opened by the removal of a slice not quite sufficiently 

 large to bisect these structures. 



The appearance with the slice removed is given in fig. 14. The conus was found to 

 be perfectly straight, leading into a relatively much larger ventricular cavity than we 

 find in the Dipnoi. In fact there seems little room for doubt that the cavity of the 

 ventricle between the auiiculo-ventricular valve and the base of the conus, as delineated 

 in Chimcera, by the change in the character of the walls represents genetically the lower 

 vertical and the transverse segments of the arterial cone of Dipnoi. I am inclined to 

 think that it is only the upper vertical segment of the cone of Dipnoi which should be 

 compared with the outstanding " cone " of the Chimsera's heart. And when these two 

 portions are compared in Cerafodus and Chimcera the resemblance is found to be exact. 

 Chimcera (the specimen at least examined by me) has two transverse rows of watch- 

 pocket valves in the conus, four in each row, precisely as in the upper limb of my 

 specimen of Ceratodiis. The agreement extends to the longitudinal pads which exist 

 below the uppermost or larger series of valves, but not below the lower and smaller 

 row. In fig. 14 only one complete and two cut valves in each row are seen, the rest 

 of the wall of the cone and ventricle having been taken away altogther, and not merely 

 pinned back (as in the dissections of Ceratodus). 



If, then, we may regard that part of the ventricular cavity to the left in my drawing as 

 what in the Dipnoi becomes drawn up out of the heart and developed as an addition to the 

 existing cone, we may look in that part of the cavity to the extreme right (the animal's 

 left) for indications of parts corresponding to the auriculo-ventricular curtain valve and 

 the basilar fibro-cartilage. I think that it is possible to recognize these in the com- 

 ponents of the tricuspid valve by which the auricle and ventricle of the Chimsera's 

 heart communicate. 



In order to understand the form of this valve it must be looked at first of all from 

 the auricular side ; and it is so seen in fig. 15, A, AW. A hole like a leech-bite is there 

 seen leading from auricle to ventricle. Three pads are placed between the three arms 

 of the opening. One of these pads (^FF) is more prominent and membranous than 

 the other two. It may be taken to represent the auriculo-ventricular curtain valve of 

 Dipnoi. Of the remaining two pieces one is larger and firmer than the other, and 



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