THE DERMAL FIN-RAYS OF FISHES. 499 



ceratotrichia of Elasmobranclis, with overlying genuine scales^ 

 and that the two elements remain separate beyond the base of 

 the fin, where they enter the body. This view will be farther 

 discussed below (p. 510). 



To conclude, it may be said that the evidence derived from 

 the study of the fossil Actiuopterygii points undoubtedly to 

 the view that the lepidotrichia are homologous with the body- 

 scales, from which they have been partially, if not entirely 

 derived. Further, it may be concluded that the primitive 

 Actinopterygian ancestors, from which the various diverging 

 groups have been derived, possessed lepidotrichia in which 

 the segments bore a very close resemblance to the scales, in 

 histological structure especially, fins pi'ovided with a complete 

 edging of fulcral scales along the anterior border (fig. 54), 

 and probably also fulcra along the dorsal edge of the tail. 

 Concerning the presence of actinotrichia at the margin of the 

 fius of the fossil forms, I have no direct evidence; but, 

 judging from analogy, it may safely be concluded that such 

 delicate i-ays were present also in the extinct Actino- 

 pterygians, and we may hope to discover remains of them in 

 sections through well-preserved specimens. 



Crossopterygii . 



For our knowledge of the structure of this very hetero- 

 geneous group we are chiefly indebted to the works of 

 Agassiz (1), Pander (27), Huxley (18, 19, and 20), Traquair 

 (34 and 37a), and A. S. Woodward (42). In the study of the 

 fin-rays we meet with much greater diversity of form amongst 

 these fish than amongst the Actinopterygians. 



Real fulcral scales are not found in the Crossopterygii; 

 nevertheless the dermal fin-rays at the front edge of the fins 

 are often stronger and thicker than the others, and specialised 

 large scales may be found at the base of the fins (figs. 50 and 



51). 



Osteolepidee. — In this family the lepidotrichia resemble 

 vei'y closely those of the Palaeoniscidae. We find here the 



