PISCES 131 



The notochord is replaced by bony vertebrae which are hour-glass- 

 shaped. 



2. The skull (Fig. 70, D) is covered above and below by numer- 

 ous dermal investment bones that are much Uke those of the primitive 

 extinct Amphibia {Stegocephali). 



3. The integument is covered by heavy rhomboid scales that 

 are plated externally with a sheet of ganoin. 



4. The pectoral fins are lobose in outline, but in their skeletal 

 parts show evidences of relationship to the pentadactyl limb. The 

 basal bones are homologous with the finger, wrist, and arm bones of 

 Amphibia, and the fringe of dermal rays is the aquatic part of the 

 appendage. 



5. There is a persistent spiracle, like that of the elasmobranchs. 



6. The intestine has a spiral valve. 



7. The conus arteriosus has several rows of valves. 



8. The median fin system is essentially continuous frorn dorsal 

 to ventral side, though a large part of the anterior dorsal end of it is 

 broken into separate spines each with a flap of skin back of it. The 

 caudal fin is diphy cereal. 



9. The pelvic fins are much reduced. 



10. The air-bladder is double and opens on the ventral side of 

 the pharynx. 



11. The lower jaw is sheathed in dermal investment bones, the 

 dentary and angulare. 



Some of the extinct Crossopterygii were more highly specialized in 

 certain respects than Polypterus. They have the median and paired 

 fins more like those of modem teleosts, overlapping scales, and either 

 heterocercal, or a modified type of the latter called gephyrocercal, 

 caudal fins. They are, however, more primitive than Polypterus in 

 having persistent notochord and acentrous vertebrae. 



On the whole then Polypterus may be said to be the most general- 

 ized teleostome fish living and may well be considered as the prototype 

 of that group. 



Order II. Chondrostei (Cartilaginous Ganoids) 



The Chondrostei and Holostei of the ganoid orders, and the Tele- 

 ostei, together constitute a division known as Actinopterygii, in which 

 the paired fins are sharp instead of lobate as in the Crossopterygii. 

 The modem representatives of the Chondrostei are the Paddle-Fish 



