542 BERTRAM G. SMITH 



more or less solid substratum, the endoskeletal elements of the 

 limb.'; become greatly strengthened and progressively longer 

 in order to lift the body from the ground. Such a progressive 

 elongation of the limb bones, particularly the proximal elements, 

 may be traced in both fossil fishes and amphibians ; in the latter 

 the pelvic girdle is also found becoming definitely articulated 

 with the axial skeleton. The ancestral form must have been 

 short -bodied; for in many groups of animals a progressive elonga- 

 tion of the body, culminating in eel-like forms, is found to accom- 

 pany a degenerate structure scarcely capable of giving rise to 

 higher forms (Gregory '07, Appendix I). 



The lack of scales with the potentiality of fusing into bony 

 plates is alone sufficient to exclude the elasmobranchs from the 

 immediate ancestry. For affinities ancestral to the amphibia 

 most authors have looked to the crossopterygii or the dipnoi. 

 Both have dermal bones, and both fulfil the requirement regarding 

 fins with widely protruded basal lobes and with endoskeletal 

 elements from which the framework of true limbs might be 

 derived. 



At first sight the dipnoi seem best to bridge the gap between 

 fishes and amphibia. For the lung-fishes have survived bj'- 

 virtue of an approach to the tetrapod type, enabling them to 

 exist during periods of drought. But various considerations 

 derived from the study of paleontology and comparative anatomy 

 make it probable that these terrestrial adaptations were independ- 

 entl}'- acquired, and that the dipnoi were already too highly spe- 

 cialized in other respects to give rise to the amphibia. The mosaic 

 of small bones forming the greater part of the roof of the skull, 

 particularly in the fossil representatives of this group (e.g., Dip- 

 terus), and the usual occurrence of one to several large median 

 elements in this region, make it difficult or impossible to homol- 

 ogize the dermal bones of the skull with those of amphibia. The 

 characteristic dentition is far removed from that of the amphibia. 

 Marginal teeth, with exceptions in the cases of some very earlj^ 

 forms (e.g., Phaneropleuron) , are lacking; in the later forms 

 the loss of maxillae, premaxillae and nasals shows a progressive 

 tendency toward degeneration in these regions. The concen- 



