98 



VERTEBRATE ZOOLOGY 



example of this group is the sun-fish, representing that vast assem- 

 blage of modern fishes, the Acanthopterygii, which includes more 

 species than all other groups of fishes combined. Extreme cases of the 

 laterally compressed type are seen in the "Head-Fishes" and in Zan- 

 clus (Fig. 86), which are about as high as they are long and are very 

 much compressed. This foreshortening of the long axis accompanied 



FIG. 52. Origin and Adaptive Radiation of the Fishes. Dotted areas rep- 

 resent groups still existing; black areas represent extinct groups. (After Osborn 

 and Gregory.) 



by corresponding increase in height may be taken as a symptom of 

 racial senescence; for, according to this view, there has been a retarda- 

 tion in growth vigor down the principal axis accompanied by a marked 

 acceleration of growth in the secondary (dorso-ventral) axis. 



The bottom-feeding type (Fig. 53, j, k, I) is one that involves many 

 grades and types of specialization. In general, bottom feeders are 

 depressed dorso-ventrally and are broad bilaterally. Common rep- 

 resentatives of this adaptive assemblage are the skates and rays, the 

 extinct ostracoderms, and several types of bottom-feeding teleosts. 

 They are essentially the oldest or most senescent of the fish types, 

 as evidenced by the comparative suppression of the primary or longi- 

 tudinal axis and the secondary or dorso-ventral axis by the tertiary 

 or bilateral axis. 



Thus it will be seen that the fishes, although in a less varied habitat 



