26 PISCES. 



" archipterygium" of Gegenbaur, such as survives in the modern 

 Ceratodus. The later sharks, as is well-known, have much 

 more shortened pectoral supports. Again, it may be inferred 

 from the elongate form of the lobe in the paired fins of the 

 Crossopterygian Holoptychius, that these appendages Avere on 

 the plan of the Gegenbaurian " archipterygium " ; while it is 

 known that the shortened lobate fins of the Rhizodontidse, 

 which became dominant after the Holoptychiidse had passed 

 away, were supported by a partially atrophied " archipterygium " 

 (fig. 23) ; and the short lobate fin of the modern Polypterus 

 is almost or quite tribasal, as in a typical existing shark. 



The endoskeletal fin-supports are never correlated with the 

 filiform dermal rays (actinotrichia) in the Elasmobranchs, and 

 very rarely among the Crossopterygian s (e.g., tail of Coelac- 

 anthidse, dorsal fin of Polypterus, in which each dermal ray may 

 represent a fused cluster of actinotrichia). The same is true of 

 the paired fins of the higher and later fishes, which begin with 

 types on the level of the Chondrosteans, or sturgeons and their 

 allies. In the latter fishes, however, the median fins are 

 never subdivided by concrescence of the supports; the mem- 

 brane seems to degenerate first, the supports afterwards. It 

 is in these that the correlation of the supports with the 

 dermal rays (probably now fused clusters of actinotrichia) is 

 gradually and normally developed. All the known Palaeozoic 

 fishes with paired fins as abbreviate as those of the sturgeons 

 (except one Permian genus), exhibit the median fin-supports in 

 two series and much fewer than the dermal rays. A very 

 small number of these survive in the Mesozoic and have allies 

 at the present day ; but all other post-Palaeozoic " Actinoptery- 

 gian" fishes and the Permian AcentropJiorus exhibit only a 

 single well-developed series of median fin-supports, which equal 

 in number the dermal rays apposed to them, and the second 

 series is never represented by more than insignificant remnants. 



According to the condition of the paired fins the orders of 

 the four great sub-classes of fishes may thus be arranged in 

 parallel columns as follows : 



