172 VERTEBRATE ZOOLOGY 



it bears evidences of senescence. Hence it is not at all the kind of 

 group which we would expect to give rise to the generalized verte- 

 brates of the early period. It is rather to be thought of as a preco- 

 ciously specialized, bottom-feeding derivative of some early fish-like 

 group. 



ARTHRODIRA 



The Arthrodira are real armored Fishes, whose exact relations are 

 unknown. It is not unlikely that they represent an adaptive radia- 

 tion from the primitive lobe-fin ganoids. They are provided with a 

 heavy cephalic and a separate thoracic shield and have a typical fish 

 body of primitive structure. The genus Coccocteus (Fig. 100) is a good 

 example of the group. 



