224 IOWA GEOLOGICAL SURVEY 



Lower dental plates subtriangular in outline, functional sur- 

 face more or less transversely arched and marked by eight very 

 strong subacute ridges, rather coarsely tuberculated or denticu- 

 lated in the unworn condition, but becoming smooth in the effete. 

 The antero-internal margin is formed by, the most prominent 

 of these ridges, which is at the same time more produced in front 

 and more sinuous than the others, and sometimes more coarsely 

 tuberculated. Slightly worn specimens often have the denticles 

 bordering the external margin of large size, conically pointed 

 and well separated. 



Upper dental plates agreeing in outline with the lower, and 

 having an equal number of tuberculated costae which radiate 

 outwardly in gently sweeping curves from the postero-internal 

 angle (D. flabelliformis type). Triturating surface nearly or 

 quite flat, sometimes even slightly concave. Owing to the tenuity 

 of the upper plates, their delicate external margin is very liable 

 to injury, and for that reason is seldom preserved entire. The 

 best examples known to Newberry, those figured in Plate 27, 

 figures 21 and 21a of his Monograph on Palceozoic Fishes, are 

 defective in this respect, hence it is scarcely to be wondered that 

 their association in the same mouth with the type of D. nelsoni 

 was entirely unsuspected by him. The teeth designated as D. 

 flabelliformis by this author are merely imperfect specimens 

 of the same form as is shown in Plate VII, fig. 1 of the present 

 work; and the latter clearly reveals itself as a palatal dental 

 plate agreeing in all essential respects with the mandibular ele- 

 ments figured alongside it in the same row (Plate VII, figs. 2-4). 

 One of the latter, the original of figure 2, is the actual type 

 serving for Newberry's figure and description of D. nelsoni. 

 Collected in the first instance by the late Professor Beecher. it 

 is now the property of the Peabody Museum at Yale University, 

 and has been generously loaned for study by Professor Charles 

 Schuchert, of that institution. The remaining examples of this 

 species figured in Plate VII belong to the collections of the 

 Museum of Comparative Zoology. All are from the Chemung 

 group of Warren county, Pennsylvania. In the same beds with 

 these teeth are found numerous punctate scales, eeratohyal 

 bones and other detached hard parts, including the calcified 



