DEVONIAN FISHES OF IOWA 237 



much greater significance than would otherwise be possible. The 

 osteology of various typical members of the Crossopterygian 

 order is now quite satisfactorily known, as the result especially 

 of the researches of Pander,* Huxleyf and Traquair. To the 

 last named author^ also, we owe our principal knowledge of the 

 structure of Palaeoniscid fishes, these being the only Devonian 

 representatives of the next higher order of Teleostomes, or 

 Actinopterygii. 



Order CROSSOPTERYGII. 



Pectoral fins lobate, with a large basal portion covered with 

 scales, more or less fringed with dermal rays ; the ventrals usu- 

 ally much like the pectorals, always abdominal in position; 

 caudal fin diphycercal or heterocercal. Skeleton more or less 

 ossified, body covered with rhombic or circular ganoid scales. 

 A pair of large jugular plates developed in place of branch- 

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

 into many finlets. 



Attention has frequently been called to the fact that the ear- 

 liest members 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 are further interesting 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 Smith "Woodward: (1) The paired fins become abbre- 

 viated; (2) the supports of the median fins tend towards reduc- 

 tion to a single series; (3) these supports sometimes become 

 correlated in part with 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 be- 

 coming lost. The reduction in the complexity of the mandibu- 

 lar ramus is also especially noteworthy. 



* Pander, C. H. Ueber die Saurodipterinen, Dendrodonten, Glyptolepiden, und 

 Cheirolepiden des devonischen Systems. St. Petersburg, 1860. 



t Huxley, T. H. Illustrations of the Structure of the Crossopterygian Ganoids. 

 Mem. Geol. Sur. United Kingdom, 1866. Reprinted also in the Scientific Memoirs 

 of T. H. Huxley, Suppl. volume, 1903. 



t Traquair, R. H., Ganoid Fishes of the British Carboniferous Formations. 

 Monogr. Palfeontogr. Soc. 1877, 1901, and 1907. 



