THE EVOLUTION OF FINS. 



33 



observed in the differentiated median fins of the earUer fishes. 

 Between the extremities of the unaltered parallel bars (r.) there are 

 the remnants of similar bars {ic.) that have evidently been reduced 

 and displaced by growth pressure, just as a middle digit sometimes 

 becomes reduced and displaced from connection with the carpus in 

 Ichthyosaurus, some marine mammals, and birds. Most of the car- 

 tilages bifurcate a little distally, but that is a minor matter. The 

 segmentation of the rays, the persistence of one of the middle rays, 

 with the concomitant partial fusion of the still further crowded and 

 reduced bordering rays, would soon, in the writer's opinion, result in 

 the " archipterygium " of Gegenbaur. It is, moreover, significant 



Fig. 7.— Pectoral fin of Xeitacanthus decheni, showing dermal rays. After A. Fritsch. The left 

 border is preaxial, the right border postaxial. In this interpretation the present writer is 

 directly opposed to Dr. Fritsch. 



that the anterior (preaxial) rays are much more robust than the 

 posterior (postaxial) rays, exactly as in all known examples of the 

 " archipterygium." 



Whether or not the biserial " archipterygium " results from the 

 crowding of the parallel cartilaginous rays as now suggested, or 

 whether it arises much like the dorsal fin of Holoptychius (Fig. 4), as 

 supposed by Dr. Anton Fritsch (Fig. 8), Palaeontology appears to 

 have demonstrated that this is the. type of paired fin from which all 

 later forms have been derived by abbreviation. Among Crosso- 

 pterygian Ganoids, the Holoptychiidae are admitted to rank below the 



