244 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 34 



The Acanopterygii are already much better adapted to free swim- 

 ming by the structure of their paired fins, which can be closely ap- 

 plied to the body. But in some of the oldest Chondrostei (fig. 11, C ; 



F.r.fRE r^— Gradual evolution of ossiflcation of tlie internal skeleton in different 

 Teleostomi; A, in a Crossopterygii (TJiursius) from the Devonian; B, in a Chrondro- 

 st.i (Palaconiscm) from the Permian: C. in a Holostei (DapcdUis) from the 

 Jurassic; D, in a Teleostei (Hoplopteryx) from the Tertiary. (After Woodward.) 



fig. 12, B) which reach their greatest abundance in the younger 

 Pdeozoic, the inner skeleton is still weakly ossified and the body 

 covered with thick, heavy, ganoid scales, and the tail heterocercal 

 with a long, scaly axis running out into the upper lobe. 



