VERTEBRATA 



3S 



Podopterygia. They are deficient in various normal ossifi- 

 cations, and have an additional series of membrane bones 

 in the middle line of the skull. The two families are the 

 Acipenseridse, or sturgeons, and the Polyodontidse, or 

 paddle-fishes. Both are represented at the present day in 

 the northern regions of both hemispheres, and both appear 

 first in geological time in the Eocene system. 



Superorder Actinopterygia. 



In this superorder we have the finally specialized type 

 of the true fishes. This consists in the abbreviation of the 



skeletal parts of the true fins, 

 so that the basilar elements 

 become sessile on the scapula. 

 (Fig. 10.) Coincidentally with 

 this result, the fin-rays of the 

 median fins become distinct- 

 ly developed and articulated 

 each with its corresponding 

 baseost or axonost. Fishes 

 of this superorder have, for 

 the most part, homocercal 

 tails, the superior lobe being 

 contracted to the size of the 

 inferior lobe, and the verte- 

 brae ossified; but in some of 

 the lower and older types 

 both heterocercal tails and 

 notochordal vertebi'te still re- 

 main. Diphycercal tails also continue well along the as- 

 cending scale. 



The Actinopterygia fall into two tribes : 

 Ventral fins abdominal; a ductus pneumaticus; 

 no spinous dorsal fin; parietal bones not 

 usually separated by supraoccipital ; scales 

 usually cycloid ; Malacoptcrygia. 



Jlaf 



Fig. va.—Salmo fario L ; left shoulder- 

 girdle; Cm, posttemporal ; D 1, epiclavi- 

 cle ; D, clavicle ; D 2, postclaricle ; S, 

 scapula : Co (CI), coracoid; Ba, basilars; 

 L, scapular foramen ; US, FS, flin-rays. 



