DEVONIAN FISHES OF IOWA 255 



Order ACTINOPTERYGH. 



Paired fins nonlobate, having an extremely abbreviated endo- 

 skeletal portion, and the dermal rays prominent. Caudal fin 

 abbreviate-diphycercal, heterocercal, or homocercal. A single 

 paired series of transversely elongated rays, with or without an 

 anterior azygous element, developed in the branchiostegal mem- 

 brane between the mandibular rami. 



Suborder CHONDROSTEI (Sturgeons). 



In these fishes, the oldest and most primitive of the Actinop- 

 terygii, the notochord is more or less persistent, the supports 

 of the dorsal and anal fins are less numerous than the dermal 

 rays opposed to them, the paired fins more abbreviate than in 

 the Crossopterygian order, and the tail completely heterocercal. 

 Primitive Sturgeons differ also from fringe-finned ganoids in 

 the development of a paired series of transversely elongated 

 branchiostegal rays to replace the pair of jugular plates be- 

 tween the mandibular rami; infraclavicular plates, however, are 

 retained in both groups. Nearly all the older forms have a well 

 developed rhombic and ganoid squamation. So far as known 

 the chondrocranium is but little ossified, and the cranial bones 

 are mainly dermal. 



The evolutionary history of the Sturgeon tribe, and the lead- 

 ing characters of the family Palasoniscidae, are thus indicated by 

 Professor Bridge in the Cambridge Natural History volume on 

 Fishes (1904), p. 485: 



"The Chondrostei are first represented in the Lower Devonian 

 by the solitary Palaeoniscid genus Cheirolepis, a contemporary 

 of the earliest Crossopterygii. They occur throughout the 

 Mesozoic period, except in the Cretaceous, and also in the 

 Eocene, and while steadily diminishing in number and variety, 

 they gradually approximate to their degenerate and in some 

 respects highly specialized descendants, the sturgeons and pad- 

 dlefishes of the existing fish-fauna. Of the seven families in- 

 cluded in the group, the Palseoniscidae are the oldest and most 

 generalized. The Platysomidae are a specialized offshoot from 

 the Palseoniscidae, and, if they are rightly to be considered as 

 Chondrostei, perhaps the same may be said of the problematic 



