508 EDWIN S. GOODRICH. 



mediate forms uniting the one with the other. As we pass 

 from Dipterus to Scaumenacia we see the dermal rays 

 becoming longer, more slender, and more even in size. The 

 segments also become longer and of approximately equal 

 length throughout. In Ceratodus the rays become irregu- 

 larly jointed and less branched; they tend to lose the calci- 

 fication and to become more flexible and fibrous. Finally, in 

 Protoptei-us the rays are cylindrical, fibrous, almost quite 

 uujointed and unbranched structures, with little calcification 

 and fewer bone-cells. The rays of Protopterus and Lepido- 

 siren approach, in some respects, the ceratotrichia of Elasmo- 

 branchs. 



The transition is so gradual, and the relation of the 

 proximal end of the dermal rays to the endo-skeleton and to 

 the superficial covering of scales is so similar in all cases, 

 that there can be little doubt that the dermal rays of these 

 Dipnoi are homologous structures, to all of which the name 

 camptotrichia may conveniently be applied. 



The two next questions are too intimately connected to be 

 treated separately. 



Either we may assume that the camptotrichia of the 

 Dipnoi are homologous with the ceratotrichia of the Sela- 

 chians, and that the superficial covering of fin-scales corre- 

 sponds to the basal plates of the denticles, and therefore also 

 to those scales which have become converted into the lepido- 

 trichia of the Teleostomes (p. 490) ; or we may suppose that 

 the camptotrichia of the Dipnoi are homologous with the 

 lepidoti'ichia of the Teleostomes, and have become covered 

 over by a secondary extension of the scales of the body, as 

 appears to have happened in the case of some Teleosts (p. 499). 



On the first hypothesis the dermal rays of Protopterus or 

 Lepidosiren would naturally be considered as the most 

 primitive in the Dipnoi, since they most closely resemble the 

 ceratotrichia. The rays of Scaumenacia and Dipterus would, 

 on the contrary, be amongst the most specialised. Now if 

 we appeal for evidence to the other parts of these fish, we 

 find that their testimony points to an almost diametrically 



