VERTEBRATA 



29 



Subclass III.— DIPNOI. 

 Two orders of this subclass are known, as follows: 



Paired fins rudimentary 

 or absent; pelvic ele- 

 ments (so far as 

 known) double, lat- 

 eral ; body more or 

 less protected by 

 plates ; Arthrodira. 



Paired fins archipterygial 

 (;'. e., unibasal); pel- 

 vic elements fu$ed 

 -?j- on the middle line; 

 body without bony 

 plates (Fig. 8); Sirenoidea. 



The Arthrodira are represent- 

 ed by three or four families, the 

 Coccosteidse, the Asterosteidse, and 

 the Mylostomidse, all restricted to 

 Kr^ft; %7SZti'"iX u^Tbtai ^^^ P^riods of the Dcvouic sys- 

 llS!As'^>%i^tnotrt^iTkok tem. Their remains abound in 

 wiedersheim. ^j^^gg horizous of Europc and North 



America. They possess, behind the head, a segmented 

 dorsal plate and a compound ventral plate or plastron. 

 The anterior lateral superior plates articulate with the 

 posterior lateral cranial elements by a hinge-like joint. 

 Notochord persistent. These forms have some resemblance 

 to the Agnatha Antiarcha, but they have a well-developed 

 mandibular arch, and distinct pelvic elements. Their teeth 

 are processes of the edges of the jaws. 



The Sirenoidea commence in the Carbonic system and 

 continue to the present day, when representatives of two 

 genera, Ceratodus and Lepidosiren, exist in the fresh 



