3i6 Evolution and Distribution of Fishes 



CHAPTER XL 



The Evolution of Fishes. III. The Chondrostei and 

 Protospondylei ( Holostei ) . 



The Chondrostei. The second great group of the Tele- 

 ostomi or higher fishes ranks next in time of appearance 

 and structure to those already examined. That the group 

 is more advanced and highly evolved than those already 

 studied is proved: (a) by the known presence only of one 

 genus in Old Red rocks; (b) by the more perfect ossifi- 

 cation of the skeleton; (c) by the condensed character of 

 the supports for the paired fins; (d) by the tail-fin being 

 rarely diphycercal, usually heterocercal, sometimes homo- 

 cereal. 



A. S. Woodward's introductory observations on the 

 group in Volume 3 of his "Catalogue" are so exact that we 

 quote as follows: "The origin of these fishes is still en- 

 tirely obscure. Among known fossils they range down- 

 wards as far as the Crossopterygians, while there is as yet 

 no evidence of a link between these two groups. On the 

 other hand it is clear that the Chondrostean is later than 

 the Crossopterygian type; for the former is represented in 

 the Devonian solely by the rare genus Cheirolepis, while 

 the latter is dominant throughout, and the members of the 

 Chondrostei do not flourish vigorously until those of the 

 Crossopterygii begin to decline in the Carboniferous and 

 Permian. The modifications by which a Crossopterygian 

 could be changed into a Chondrostean are also readily com- 

 prehensible. In the latter the paired fins are always poly- 

 basal, with excessively shortened lobe; and among Cros- 

 sopterygii the genera with most elongated lobate fins 

 flourish the earliest, all survivors above the Devonian hav- 

 ing the lobe comparatively abbreviate." And after re- 

 viewing additional characters, he writes: "when the 

 Chondrostei suddenly become dominant, as they do in the 

 lower Carboniferous, they already exhibit a remarkable 

 series of modifications," and some of these he then dwells 

 on. 



