DEVONIC FISHES OF THE NEW YORK FORMATIONS 1 65 



and Traquair. To the last named author,' also, we owe our principal 



knowledge of the structure of Paleoniscid fishes, these being the only 



Devonic representatives of the next higher order of Teleostomes, or 



Actinopterygii. 



Order i CROSSOF»XERYaiI 



Pectoral fins lobate, with a large basal portion covered with scales, 

 more or less fringed with dermal rays ; the ventrals usually much like the 

 pectorals, always abdominal in position ; caudal fin diphycercal or hetero- 

 cercal. Skeleton more or less ossified, body covered with rhombic or 

 circular ganoid scales. A pair of large jugular plates developed in place of 

 branchiostegals ; no fulcra. Dorsal fins two, or a single one divided into 

 many finlets. 



Attention has frequently been called to the fact that the earliest mem- 

 bers of this order, especially those with acutely lobate paired fins, bear a 

 considerable resemblance to Dipnoans, and the descent of the latter has 

 even been traced by Dollo and others to the Crossopterygii. These fishes 

 possess much general interest in that they are commonly looked upon as 

 ancestral to amphibians and higher vertebrates. During the evolutionary 

 history of the order, the following structural modifications are observable, 

 as noted by A. S. Woodward: (i) The paired fins become abbreviated; 

 (2) the supports of the median fins tend towards reduction to a single 

 series; (3) these supports sometimes become correlated in part Avith the 

 dermal fin rays ; and (4) there is sometimes degeneration in the external 

 armor of the head and opercular region, some plates being fused together, 

 others becoming lost. The reduction in the complexity of the mandibular 

 ramus is also especially noteworthy, 



Family holopxychiid-A-EI 

 Body fusiform, with cycloidal, deeply overlapping scales, more or less 

 enameled. Head and opercular apparatus with well developed membrane 

 bones ; parietals large and separate ; frontals separate, not fused into a con- 



' Traquair, R. H. Ganoid Fishes of the British Carboniferous Formations. Pt i, 

 Palaeoniscidae. Pal. Soc. Monogr. 1877 and 1901. 



