8 STRTJCTrEE OF THE PAIKED FIXS OF CEEATODFS, 



several are bifurcated. The pelvic fin of the opposite side of 

 the same specimen (fig. 4) has a totally different structure and 

 very closely resembles the pectoral fins. There is a broad second 

 joint divided distally by a fissure, the posterior division being 

 connected distally with the main axis of the fin and supporting 

 posteriorly three short rays, while the anterior forms the base of 

 stout anterior ray or unbranched anterior axis. None of the 

 rays in this fin are branched and the arrangement approaches 

 very closely to that figured and described by Griinther (fig. 3.). 

 The pectoral fins of the second specimen are very similar to 

 those of the first, except that the anterior axis gives off no 

 branches. The left pelvic fin of this specimen (fig. G) somewhat 

 resembles that of the first, but there are important differences 

 in detail. Thus the anterior axis is much longer and more com- 

 plexly branched, and the main axis itself subdivides into two, 

 only one of which, however, is continued to the extremity of the 

 fin. The right fin (fig. 7), is very similar to the left fin of the 

 first specimen with some slight j)oints of diff'erence which hardly 

 require description. 



As Griinther and Huxley each examined two specimens, this 

 branching of the cartilaginous skeleton of the fin of Ceratodus 

 would appear to be an exceptional arrangement ; and it is 

 reasonable to regard it as an instance of atavism, and as pointing 

 back to a pre-existing condition in which the fin-skeleton consisted 

 of branching jointed cartilaginous elements supporting a cutane- 

 ous expansion considerably broader than that of the fin of the 

 living Ceratodus Forsteri. The second joint seems to be formed 

 by the coalescence of the bases of several of the main ribs or 

 axes of the fin, and the first joint is to be regarded as derived 

 from the equivalent of the hasipterygium of the embryonic 

 Sci/Uium. In the absence, however, of any data on the develop- 

 ment of Ceratodus the homologies of these cartilages cannot be 

 determined with any certainty, but the varieties of arrangement 

 which I have described seem to point to the above conclusion. 



