The Evolution of Fishes 



451 



vailed in Devonian times. The larvas possess external gills 

 with firm base and fringe-like rays, suggesting a resemblance 

 to the pectoral fin itself, which develops from the shoulder-girdle 

 just below it and would seem to give some force to Kerr's con- 

 tention that the archipterygium is only a modified external 



Fig. 260. — An extinct Crossopten'gian, Holoptychius giganieus Agassiz (1S35). 

 (Alter Agussiz, per Zittel.) 



gill. In Polypterns the archipterygium has become short and 

 fan-shaped, its axis made of two diverging bones with flat 

 cartilage between. From this type it is thought that the arm 

 of the higher forms has been developed. The bony basis may be 

 the humerus, from which diverge radius and ulna, the carpal 

 bones being formed of the intervening cartilage. 



The Actinopteri. — From the Crossopterygians springs the 

 main branch of the true fishes, known collectively as Actinopteri, 

 or ray-fins, those with ordinary rays on the paired fins instead 

 of the jointed archipterygium. The transitional series of primi- 

 tive Actinopteri are usually known as Ganoids. The Ganoid 

 differs from the Crossopterygian in having the basal elements 

 of the paired fins small and concealed within the flesh. But 

 other associated characters of the Crossopterygii and Dipnoans 

 are preserved in most of the species. Among these are the 

 mailed head and body, the heterocercal tail, the cellular air- 

 bladder, the presence of valves in the arterial bulb, the presence 

 of a spiral valve in the intestine and of a chiasma in the optic 

 nerves. All these characters are found in the earlier types so 

 far as is known, and all are more or less completely lost or 

 altered in the teleosts or bony fishes. Among these early 

 types is every variety of form, some of them being almost as long 



