DEVONIAN FISHES OF IOWA 221 



gin for a variable distance toward the posterior angle ; the two 

 posterior rows often rudimentary. Some of the tubercles, when 

 worn by use or by post-mortem abrasion, become elongated in 

 the direction of the ridges to which they belong, others in an 

 oblique direction. The coarseness of tuberculation, in propor- 

 tion to the size of the plates, is greater than in any other de- 

 scribed species. Apparently some diminution in number of 

 tubercles, coincident with increase in size, takes place in the 

 teeth of adult or old individuals. 



Upper dental plates of the D. flabelliformis type do not differ 

 materially from the lower as regards outline, or size and number 

 of tuberculated ridges, but their substance is usually much thin- 

 ner, the functional surface is nearly or quite plane or in some 

 cases even concave, and in the effete condition it becomes worn 

 down almost perfectly smooth. In well preserved specimens the 

 marginal contour is seen to be ovoid or subtriangular, like that 

 of the lower dental plates, but the majority of examples have 

 the external margin deficient, owing to wear, injury or poor 

 preservation, and accordingly in that respect present deceptive 

 appearances. Precisely the same faulty condition is also true 

 of the majority of "flabelliform" upper dental plates belonging 

 to D. nelsoni, as witness Newberry's figured specimens (Monogr. 

 U. S. Geol. Survey, vol. 16, pi. 27, figs. 21, 21a). "With the aid 

 of an extensive suite of material illustrating the accidental varia- 

 tions of this nature common to both species, one is justified in 

 assigning a commensurate range of diversity to other Dipterine 

 species that are affected by the same causes. This principle 

 has been adopted in preparing the following descriptions. 



Formation and locality. State Quarry beds (Upper Devo- 

 nian) ; near North Liberty, Johnson county, Iowa. 



Dipterus digitatus, sp. nov. 

 (Plate II, Fig. 6; Plate VII, Figs. 16-25) 



1907. Palatal dental plate of undescribed Dipterine species, C. R. Eastman, 

 Mem. N. Y. State Mus. 10, p. 202, pi. 4, fig. 8. 



Lower dental plates very similar to those of D. costatus, but 

 with more elevated and convex functional surface, and more 

 numerous nearly rectilinear, tuberculated costae, which extend 



