THE DERMAL FIN-HAYS OF FISHES. 509 



opposed conclusion. Dollo (9) has strongly urged that 

 Scaumenacia^ Ceratodus^ ProtopteruS; and Lepidosiren form 

 a degenerating series. The structure of tlie skull, of the fins, 

 of the scales, and of the respiratory and other internal organs 

 of the living forms, all support this view. The evidence of 

 the skull is especially strong, and has not, I venture to think, 

 hitherto been quite rightly interpreted. 



It is a question which I do not propose to discuss in detail 

 in the present paper; but it may be pointed out that the 

 series Protopterus, Ceratodus, Ctenodus, Scaumenacia, 

 approaches nearer and nearer to the Crossopterygian type of 

 skull in the conformation of its cranial roof. Scaumenacia, 

 indeed (as I have myself observed), has an almost typical 

 Crossopterygian roof, with large paired frontals and parietals 

 meeting in the middle line, and not separated by median 

 bones. So far, then, I am inclined to agree with Dollo, and 

 believe that the evidence derived from the structure of the 

 fin-rays supports his contention that the direction of speciali- 

 sation has been from Scaumenacia or Phaneropleuron towards 

 Protopterus. 



For it must be remembered that although the campto- 

 trichia of Protopterus and Lepidosiren bear a certain 

 superficial resemblance to the ceratotrichia of Elasmobi'aDchs, 

 yet they differ from them in points which may well be 

 considered as fundamental. As was shown above (p. 482), 

 they are not homogeneous, they contain bone-cells, they are 

 generally jointed, and are developed only in a single layer. 



We have seen that the Holoptychiidfe, amongst the 

 Crossopterygii, approach most nearly to the Dipnoi in the 

 structure of their dermal rays, as they do also in their general 

 anatomy. The lepidotrichia of Glyptolepis are not dis- 

 tinguishable in any important I'espect from the camptotrichia 

 of Scaumenacia or Ceratodus. So strong is the resemblance 

 that we are almost forced to believe that the dermal rays 

 of the Dipnoi are only specialised forms of 

 lepidotrichia. 



If we adopt this view, and if at the same time we hold that 



