J. K. Thacher — Ventral Fins of Ganoids. 235 



distal segmentation is wanting on the first ray. If it were not for the 

 process «, if here in Acipetiser we had nothing but the flat piece bd, 

 there could, I believe, be no hesitation on the part of anyone in 

 regarding this piece b d as formed by concrescence from the rays 1 

 and 2. But the piece a raises an indefinite suspicion. One is led to 

 think that it may possibly be the remains of something, and that 

 despite the apparent similarity, the orad part of the fin may have 

 something more in it than the aborad, and that the part a may not 

 be a simple original upgrowth of cartilage dorsad from the orad side 

 of the first ray. 



To assure ourselves that it is this last and nothing else we turn to 

 Polyodon. A view of the ventral surface of a right ventral is 

 exhibited in PI. I, fig. 3. Here the number of rays, thirteen, is 

 greater than it was in Acipenser and Scaphirhynchus. The seg- 

 mentation into three parts is to be noted, as also the absence of the 

 distal joint in the orad ray, agreeing with what we have found so 

 generally in dorsal, anal and ventral fins. There has been some con- 

 crescence, but not very much, as seen in the proximals 12-13 and 

 1-2-3-4-5. The shortening of ray 7 is worthy of remark. The rays 

 differ a little from those that form the ventrals of Acipenser and 

 Scaphirhynchus, in that in these latter the rays continue thick to their 

 ])roximal ends, and the median edge of the fin is quite heavy, where- 

 as in Polyodon the proximal ends of the rays become very thin ; so 

 that while the distal end of the proximal segment is thick and round, 

 the proximal end runs out into a thin blade. Thus also, as would be 

 expected, the large piece b d becomes, as it approaches the median 

 line, a thin lamina of cartilage. This fin lies very nearly all in one 

 plane and there is consequently no foreshortening of any of the parts. 



When we turn the fin over so that its dorsal side is presented to 

 view, we see that the structure of the skeleton differs from what it 

 previously appeared to be in a very striking, and as we shall see, 

 significant way. The rods are not simple rods, but from the orad 

 side of the distal part of the proximal segment of each ray, a blade- 

 like process runs dorsad, approaching, consequently, a direction at 

 right angles to the plane of the fin, and lying in the intermuscular 

 septa of the fin. One of the rays from the middle of the fin is 

 exhibited in PI. I, fig. 3, a. The side of the ray is presented, and the 

 process a' has been turned about its line of junction with the ray so 

 as to lie in the plane of the paper, and not at right angles to it, as it 

 naturally would. This process a' is quite thin and flexible in the case 

 of all the rays of Polyodon, while the ray itself at the point where it 



