FOSSIL CROSSOPTERYGIANS 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. 
Celacanthus, 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.— Celacanthus elegans, Newb. X 3. Coal Measures, Ohio. 
A. Position of calcified swim-bladder. 
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. 
Diplurus 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. 
