348 On the PhyJogeny of tlie Tdeostomi. 



In the Teleostomi and the Chondropterygii * the evolution 

 of the paired fins has proceeded independently, but sometimes 

 on parallel lines, from the earliest stages. The median fins 

 of the Teleostomi also tend to undergo the same modifications 

 as the paired ones, but this comparison must not be pushed 

 too far. The most primitive condition is that which we have 

 seen in the anal and ventral fins of Psephurus: (1) dermal 

 rays much more numerous than the baseosts, which form a 

 well-developed series^ attached internally to a series of 

 axonosts, the anterior of which show a tendency to fusion. 

 From this stage is easily derived that which is seen in the 

 anal fin of Eusthenopteron, or in the ventral of Polypterus 

 or ? Coccosteus, {. e. (2) dermal rays more numerous than 

 the baseosts, which are attached to a single cartilage or bone 

 formed by the fusion of the axonosts. The third stage (3), 

 in which the baseosts are rudimentary or absent and the 

 deimal rays are attached direct to the axonostal bone, is 

 exemplified in the anterior dorsal of the Coelacanthidse and 

 the ventrals of the Teleostei. 



Two conditions met with in the median fins are not 

 paralleled in the paired ones. The first is a modification of 

 stage (1) described above, and is that seen in the Teleostei, 

 baseosts small or wanting, dermal rays equal in number to 

 the axonosts. The second is derived from stage (2), and is 

 that seen in the posterior dorsal of Holoptyckius, in which 

 there is a single axonostal cartilage, whilst the baseosts are 

 numerous, crowded, and apparently subdivided, some being 

 attached to others instead of to the axonost. 



Similarly the paired fins undergo modifications which 



* Thaclier (Tr. Conn. Ac. iii. & iv. 1877) deduced the theory of the 

 similar origin of median and paired fins from their similar structure in 

 the Elasmobranchii and Ohondrostei. Balfour, from a study of Elasmo- 

 branch development, also deduced the similar origin of median and paired 

 fins. He concluded that in modern Elasmobranchii the ventral fin retains 

 in all essential respects its primitive arrangement, and that the pectoral 

 metapterygium represents the pelvic basipterygium. He also wrote : " I 

 should be much more inclined to hold that the fin of Ceratodus has been 

 derived from a fin like that of the Elasmobranchii by a series of steps 

 similar to those which Huxley supposes to have led to the establishment 

 of the Elasmobranch fin, but in exactly the reverse direction." 



I prefer these conclusions to the more recent ones of Cope and Smith 

 Woodward, who regard the fins of modern Elasmobranchii and Chon- 

 drostei as highly specialized, and I would point out that the Ichthyotome 

 pectoral must have been derived from the Pleuropteryj^ian type in the 

 same way as the paired tins of the Dipueusti from those of the Chon- 

 drostei, the axis, or metapterygium, representing the series of axonosts, 

 and not being derived from an elongate baseost. 



