PHYLOGENY. 



lOI 



of the paired fins are present and primitive, and re- 

 semble those of one of the suborders of sharks. 



The phylogeny of the Teleostomata, as indicated 

 by the fin-structure, will commence with the Crosso- 

 pterygia. From this group the Podopterygia may be 

 theoretically derived, and from these the Actinoptery- 

 gia. The Rhipidopterygia appear to be a side group, 

 not in the main piscine line. But the oldest known 

 Crossopterygia are from the Carboniferous, while the 

 Rhipidopterygia are abundant in the Devonian. More- 

 over, thesuperorder Actinopterygia, with its contracted 

 fins, may have appeared in the Carboniferous, while 

 the Podopterygia (Palaeoniscidas) certainly did so. 



The descent of the fishes in general has witnessed, 

 then, a contraction of the limbs to a very small com- 

 pass, and their substitution by a system of accessory 

 dermal radii. This has been an ever-widening diver- 

 gence from the type of the higher Vertebrata, and 

 from this standpoint, and also a view of the "loss of 

 parts without complementary addition of other parts," 

 may be regarded as a process of degeneration. 



Taking up the great division of the Actinopterygia, 

 which embraces most of the species of living fishes, 

 we can trace the direction of descent largely by ref- 

 erence to their systematic relations when we have no 

 fossils to guide us. 



The three subtribes adopted by Jordan represent 

 three series of the true fishes which indicate lines of 

 descent. The Holostei include the remainder of the 

 old ganoids after the subtraction of the Rhipidoptery- 

 gia, the Crossopterygia, and the Podopterygia. They 

 resemble these forms in the muscular bulbus arteriosus 

 of the heart, in the chiasm of the optic nerves, and in 

 the greater distinctness of the metapterygium. The 



