536 PEOF. T, W, BEIDGE OB" THE MESIAX 



It may be remarked that the precise number of radial ele- 

 ments in the mesial fins and the extent of their concrescence 

 are subject to variation in different individuals. In a much 

 larger specimen (about 8 feet in length) the number of radial 

 elements in the dorsal fin was the same as in the smaller one ; 

 but the proximal segments which had fused into single basal 

 pieces vs^ere those of the first and second, the third and fourth, 

 and the fifteenth and sixteenth. In the anal fin only the 

 proximal segments of the third and fourth had fused. The 

 figure of the dorsal fin of an Acipenser given by Thacker [5], 

 and reproduced by Mivart [6], exhibits only fifteen radial ele- 

 ments, and those represented vpith fused proximal segments are 

 the first and second, and the eighth and ninth, while the tentli 

 and thirteenth inclusive, in addition to the first, are figured as 

 wanting their distal segments. It is also evident, from a com- 

 parison of the two specimens referred to above, that the older 

 the fish the more complete is the extent to which the proximal 

 and mesial segments become ossified, and the less intimately are 

 the various radial elements related to one another. 



POLTODONTID^. 



Folyodon folium. 



The mesial fins of Polyodon are, in the main, very similar to 

 those of Acipenser, but indications of increasing specialization, 

 and of a gradual approximation to the higher Ganoids, in certain 

 minor points are not wanting. 



Dorsal fin. — The dorsal fin is supported by a series of twenty 

 radial elements (PI. XXI. fig. 1), of which the approximately central 

 ones are the longest, and the most anterior and posterior the 

 shortest. All of them are divided into proximal {p.s.), mesial (m.s.), 

 and distal (d.s.) segments, except the first and the last, which are 

 without distal segments. The proximal segment in each element 

 is about the same length as the mesial, or only slightly exceeds 

 it, and is now somewhat dagger-shaped, with a pointed inner 

 extremity and a much thicker distal portion. The distal seg- 

 ments are mere cartilaginous nodules, forming by their close 

 apposition a well-defined and continuous margin to the periphery 

 of the fin-supports, and also exhibiting a tendency to alternate 

 with the cartilaginous distal ends of the mesial segments. The 

 connexion between the various radial elements is, perhaps, less 

 intimate than in Acipenser ; only along the centre of the series, 



