DEVONIAN FISHES OF IOWA 231 



dentition of Syntketodus, they are now regarded with much 

 greater likelihood, for the reasons stated above, as belonging to 

 a distinct genus, Conchodus, in which the characteristic features 

 of Dipterus have become well-nigh obliterated. The arrange- 

 ment of dental plates in the mouth may be presumed to have 

 been the same as in Dipterus, and the same means are available 

 for distinguishing between upper and lower dentition. A posi- 

 tion may be assigned to this form intermediate between Dip- 

 terus and Synthetodus. 



Formation and locality. Upper Devonian, Johnson county, 

 Iowa. 



Genus SYNTHETODIS. 



An imperfectly definable genus, known only by remains of the 

 dentition. Apparently the upper and lower dental organs con- 

 sisted each of three coalesced plates — a pair of ovoid lateral, 

 and a smaller azygous symphysial element. The paired elements 

 are evidently homologous with the single pair normally occur- 

 ring in Ctenodipterines, agreeing as they do with the latter in 

 microscopic structure, but differing from them in that the func- 

 tional surface is entirely smooth, without even vestigial tuber- 

 cles or costae. The azygous element is not known to occur else- 

 where among Palaeozoic Lung-fishes, and may possibly be indic- 

 ative of the Sirenoid order of Dipnoans.* 



Synthetodus trisulcatus Eastman. 



(Plates IX and XI, pars) 



1896. Synthetodus trisulcatus C. E. Eastman, Ann. Rept. Iowa Geol. Surv. 

 VII, p. 112, pi. IV, figs. 1-26. 



*As to the possibility of this order being represented in the Palaeozoic, compare 

 the following suggestive hint of Newberry, found at page 88 of his Monograph of 

 1889: 



"The group designated by the name of Dipterus is so abundant and so well 

 preserved in the Devonian rocks of Scotland that its entire structure has been fully 

 made out, and we find that it was a fish having a tesselated cranium, the palate 

 teeth already described, and a fusiform body covered with strong, enameled, punc- 

 tate scales. In the Carboniferous and still higher strata, on the contrary, the fishes 

 which carried the fan-shaped dental plates must have been somewhat differently 

 constituted, for neither in the Old nor in the New World has anything like the 

 complete form of the fish been made out. In the lagoons of the coal-marshes of 

 England and Ohio, where the circumstances were favorable for the preservation 

 of even delicate structures, the teeth, usually dismembered, but occasionally 

 attached to the palato-pterygoid and the splenial bones, and portions of the tessel- 

 lated cranium, were the only parts preserved; while, as yet, in the higher strata 

 nothing but the teeth have been found." 



