44 8 



The Evolution of Fishes 



zation the shark gradually gives place to the flattened body 

 and broad fins of the ray. This group is called Tectospondyli. 

 Those sharks of this group with one ring of calcareous matter 

 in each vertebra constitute the most primitive extreme of a 

 group representing continuous evolution. 



From Cladoselache and Chlamydoselachus through the sharks 

 to the rays we have an almost continuous series which reaches 

 its highest development in the devil rays or mantas of the tropical 

 seas, Mania and Mobula being the most specialized genera and 

 among the very largest of the fishes. However different the rays 

 and skates may appear in form and habit, they are structurally 

 similar to the sharks and have sprung from the main shark 

 stem. 



FIG. 257. Star-spined Ray, Raja stettulata Jordan & Gilbert. 

 Monterey, CaL 



The Chimaeras. The most ancient offshoot from the shark 

 stem, perhaps dating from Silurian times and possibly separated 

 at a period earlier than the date of any known shark, is the 

 group of Holocephali or Chimaeras, shark-like in essentials, but 

 differing widely in details. Of these there are but few living 

 forms and the fossil types are known only from dental plates 

 and fin-spines. The living forms are found in the deeper seas the 

 world over, one of the simplest in structure being the newly dis- 



