THE DERMAL FIN-RAYS OF FISHES. 503 



The lepidotri cilia of the CaBlacanthidae are formed of normal 

 bone, like those of other Crossopterygii. 



Dipnoi. 



We owe chiefly to Giinther (12) and Traquair (34) the 

 proper delimitation of this group, by the bringing together 

 of the fossil allies of Ceratodus, which had previously been 

 associated with the Crossopterygii. Not many genera of 

 extinct Dipnoi are known, and of these only very few are 

 sufficiently well preserved to afford any evidence with regard 

 to the fin-rays. Dipterus, Scaumenacia, and the nearly allied 

 Phaneropleuron are the only genera whose fins I have had 

 the opportunity of studying in detail.^ 



Scaumenacia has well-developed fin-membranes, supported 

 by very numerous dermal rays, as shown by Whiteaves (41). 

 Speaking generally, the dermal rays of this fish and of 

 Phaneropleuron conform well to the type of ray found in 

 Ceratodus, which I have named camptotricli (p. 486). They 

 are long, slendei*, set very close together, and overlap 

 proximally the whole of the first or distal segment of the 

 endo-skeletal fin-support. The camptotrichia of Scaumenacia 

 are subdivided into two regions of approximately equal 

 length. The proximal half is in the form of a straight, 

 rounded, unsegmented piece, which is covered over by the 

 thin scales of the body (fig. 4). The distal half is often 

 branched, has numerous and fairly regular articulations, and 

 tapers gradually to the edge of the fin. As in Crossopterygians, 

 whilst the transverse section of the proximal region of the 

 dermal ray is circular, showing concentric lines of growth, it 

 becomes more and more flattened distally, and also concave 

 on its inner surface (figs. 33 a and b). 



The bony substance of the camptotrichia contains many 

 elongated bone-cells (fig. 42). Whether the body-scales 

 extend over the fins of Scaumenacia as far as in Ceratodus, I 



> I liave to thank Dr. Traquair for the generous gift of a specimen of 

 Dipterus. 



