510 EDWIN S. GOODRICH. 



tlie lejjidotricliia oE the Teleostomes were themselves originally 

 derived from scales, we are driven to admit that the scaling 

 on the Dipnoan fin outside the dermal rays is of secondary 

 and later origin, like the scaling on the head and fins of 

 certain Teleostean fish (p. 491). We should then have two 

 generations of dermal structures superimposed one above the 

 other. That this interpretation is not improbably correct is 

 evident when we reflect that in Ceratodus itself the dermal 

 bones of the skull are, to a great extent, covered over by 

 superficial scales similar to those on the body and fins. 



To assign Dipterus to its proper taxonomic position is at 

 present a most difficult matter. Judging from the structure 

 of its cranial roof, it cannot be placed between Scaumenacia 

 and the Crossopterygii. In Dipterus the frontals and 

 parietals have apparently been reduced in size and pushed 

 aside to accommodate the newly developed median bones. 

 Were it not for its median fins, which in number and shape 

 so closely resemble those of the Teleostomes, Dipterus would 

 find a place near Ctenodus as a specialised offshoot from the 

 main Dipnoan stem, in which the dermal skeleton had 

 acquired a peculiai'ly thick coating of ganoine. According 

 to the view set forth above, whilst the dermal rays of Dipterus 

 are still quite simihir to those of the Crossopterygii, the 

 secondary outer scaling has been developed on the fins to 

 a greater extent than in any other Dipnoan. 



We have just examined the evidence in favour of the view 

 that the Dipnoan camptotrichia were derived from lepido- 

 trichia such as are found in the Teleostomes; we have also 

 briefly disposed of the theor}^ that the camptotrichia of the 

 living Dipnoi were directly evolved from the ceratotrichia of 

 Elasmobranchs. Let us now discuss a third possibility, that 

 the dermal rays of the Teleostomes and Dipnoi are formed 

 by the combination of the Elasmobranch ceratotrichia with 

 the bony superficial scaling. 



It may well be urged that the Dipnoi diverged from 

 the Teleostome stem at a very early period indeed ; that, 

 consequently, those Teleostomi which most closely resemble 



