34 COPE 



Ventral fins usually thoracic or jugular ; 

 nO' ductus pneumaticus; usually a 

 spinous dorsal fin; parietal bones us- 

 ually separated by the supraoceipital ; 

 scales usually ctenoid ; Acanthopterygia. 



In the cartilaginous fin-rays and posterior ventral fins 

 the Malacopterygia more nearly approach the fishes of the 

 superorders considered in the preceding pages, than do the 

 Acanthopterygia. The persistence of the communication be- 

 tween the swim-bladder and the gullet (ductus pneumaticus) 

 is another indication of this affinity. Accordingly we find 

 representatives of the former in older periods than we do 

 the latter. They are abundant in the Jurassic and Cretacic 

 and later periods, while the Acanthopterygia are very rare 

 in the Cretacic, and are abundant first in the Eocene. 



The orders of the Malacopterygia are the following : 



I. Median fins with actinotrichia. 



Basilars of median fins well developed ; 



notochord persistent ; (1) Docopteri* 



II. Median fin-rays equal to and articulating with 



axonosts. 



A. Metapterygium present ; anterior vertebrse unmodi- 

 fied. (Holostei). 



a, Vertebrse complex, the pleurocentra 

 and intercentra distinct. 



Anterior vertebrse similar to others ; (2) Merospondyli. 



aa, Vertebrse with centra and intercentra complete 

 on part of the column at least ; amphicoelous ; 



Anterior vertebrse similar ; (3) HalecomorpM. 



aaa, Vertebrse (intercentra) opisthocoelous. 



* The position of this order is uncertain, 



