170 NEW YORK STATE MUSEUM 



Order ACTINOPXERYGII 



Paired fins nonlobate, having an extremely abbreviated endoskeletal 

 portion, and the dermal rays prominent. Caudal fin abbreviate-diphycercal, 

 heterocercal, or homocercal. A single paired series of transversely elon- 

 gated rays, with or without an anterior azygous element, developed in the 

 branchiostegal membrane between the mandibular rami. 



Suborder CHONDROSTISI (Sturgeons) 

 In these fishes, the oldest and most primitive of the Actinopterygii, 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 are 

 more abbreviate than in the Crossopterygian order, and the tail is com- 

 pletely heterocercal. Primitive sturgeons also differ from Crossopterygian 

 fishes in the development of a paired series of transversely elongated bran- 

 chiostegal rays to replace the pair of jugular plates between the mandi- 

 bular rami ; infraclavicular plates, however, are retained in both groups. 

 Nearly all of 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 has been thus briefly 

 recapitulated by Professor Bridge in the following paragraph : ' 



The Chondrostei are first represented in the Lower Devonic 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 paddle- 

 fishes of the existing fish fauna. Of the seven families included in the 

 group, the Palaeoniscidae are the oldest and most generalized. The Platy- 

 somidae are a specialized offshoot from the Palaeoniscidae, and, if they are 

 rightly to be considered as Chondrostei, perhaps the same may be said of 

 the problematic Belonorhynchidae. On the other hand, there are certain 

 features which indicate an approach to fishes of an altogether more 

 modern type. Finally, the Chondrosteidae represent a stage in a career of 



'Fishes, Ascidians, etc. Cambridge Natural History. 1904. p. 487. 



