THE DERMAL FIN-EAYS OF FISHES. 507 



(fig. 40). Transverse sections oE the proximal unsegmented 

 region show that the bone is deposited in circular concentric 

 layers, as in allied forms (fig. 32). 



There are soaie specimens of Dipterus in which the 

 ganoine is well preserved on the body-scales ; and^ as a rule, 

 the fins in these fossils present a different appearance. 

 Instead of a mere edging of scales at and near the margin of 

 the fins, we find that the whole web of the fin is entirely 

 covered with scales, similar to the marginal scales described 

 above (figs. 57). These fin-scales have a pitted and ganoid 

 surface, and are set in regular branching rows end to end. 

 They are smaller distally than proximally, where they pass 

 gradually into the ordinary form of scale covering the body. 

 So far as my experience goes, in such fossils the jointed 

 dermal ray itself cannot be seen. In fact, when the dermal 

 rays are cleai^ly visible the superficial scales are only seen 

 near the margin of the fins; and vice vers4, when the 

 general covering of scales is preserved the dermal rays do not 

 appear. This peculiar condition of things may be due to 

 some original difference between the various specimens of 

 Dipterus, or, as seems much more probable, to a difference in 

 the preservation of the fossils. Puzzles of this nature are 

 familiar to those who study delicate fossils in detail. 



It may, I think, be safely concluded that Pander was right 

 in describing the fin-rays of Dipterus as covered with inde- 

 pendent and more superficial scales, as shown in Text-fig. 6, 

 p. 506. 



Several important questions now suggest themselves. 

 Firstly, are the jointed dermal rays homologous throughout 

 the Dipnoi ? Secondly, which is the more primitive condi- 

 tion, that of Dipterus or that of Protopterus ? Thirdly, are 

 all or any of the dermal rays of the Dipnoi homologous with 

 those of other fish ? 



With regard to the first question, it may be pointed out 

 that Dipterus on the one hand, and Protopterus (or Lepido- 

 siren) on the other, represent the two extremes of structure 

 in. the dermal rays. Scaumenacia and Ceratodus are inter- 



