FOSSIL CR OSSOPTER YGIANS 



153 



tion has been noticed in Fig. 43. The condition of the 

 caudal fin of Eusthenopteron is also worthy of note ; the 

 tip of the notochord is retained although the functional 

 portion of the fin is derived from the more anterior 

 body region. The vertebral arches are here clearly sug- 

 gestive of the conditions of the Dipnoan. 



CoelacantJms, common in the Coal Measures (Fig. 155), 

 is the most specialized of the Crossopterygians ; it has 

 retained all of the archaic structures of its kindred, yet 

 has concealed them under the outward appearance of a 

 recent bony fish ; the general contours of its head, trunk, 

 scales and fins resemble strikingly those of a dace or 



Fig. 155. — Caelacanthus elegans, Newb. 

 A. Position of calcified swim-bladder. 



X ?. Coal Measures, Ohio. 



chub ; but on closer view the paired fins are found to be 

 archipterygial, the scales enamelled and sculptured, the 

 true caudal fin the degenerate stump of the notochord ; 

 the functional caudal has been formed of the enlarged fin 

 rays of the dorsal and anal region. Traces of a calcified 

 air-bladder, A, are often preserved. 



Diphirus and a closely related genus, Undina (Figs 156, 

 156 A), may finally be noted among the highly evolved 

 Crossopterygians. They appear in the Mesozoic when the 

 majority of their kindred have disappeared ; they have as- 

 sumed peculiar characters and have apparently reached the 

 point of differentiation when they shortly become extinct. 



