196 IOWA GEOLOGICAL SURVEY 



lines from Coccosteus, and characterized by the persistence of 

 this distinctly Coccosteus-like feature. We should therefore be 

 inclined to look upon D. halmodeus as standing in the same an- 

 cestral relations to the type species of this genus as does D. pus- 

 tulosus to D. terrelli. It is noteworthy that D. pustulosus does 

 not occur in rocks to the eastward of Kentucky, so far as known, 

 until very late in the Devonian (Oneonta beds) ; whereas in the 

 Mississippi Valley region it is tolerably abundant throughout the 

 Middle Devonian. Its advent, then, in the Hamilton limestone 

 or its equivalent in the central western states is probably to 

 be explained on the theory of immigration from Eurasia by way 

 of the Mackenzie Basin and Manitoba.* 



The arrangement of cranial roofing plates in the headshield of 

 this species is shown in the accompanying text-figure 28, along- 

 side of which is placed, for sake of comparison, one showing 

 the corresponding portion of the existing Neoceratodus. Due 

 allowance being made for the fact that the preorbital plates 

 remain cartilaginous and even pierced by a fontanelle in the 

 recent form, and that the anterior median element (''dermal 

 mesethmoid") is undivided, as it is also in Macropetalichthys, a 

 general similarity in pattern will not fail to be observed. The 

 significance of the various points of agreement can hardly escape 

 notice when comparisons are made between other parts of the 

 skeleton as well. That which is important to bear in mind is that 

 the Arthrodiran skull, as exemplified by the best known genera, 

 was constructed upon essentially the same model as in Neocer- 

 atodus; and the latter, accordingly, serves as a most valuable 

 criterion for interpreting structural details of members of the 

 extinct group we are considering. 



Formation and locality. Fragmentary remains of this species 

 are not uncommon in the Cedar Valley limestone of Bremer and 

 Johnson counties, Iowa, and the excellently preserved headshield 

 shown in Plate IV was obtained from the same formation near 

 Bock Island, Illinois. The original of Plate III, fig. 12, was also 

 derived from the latter vicinity. The typical locality for this 

 species, where it seems to have flourished in considerable nu- 



*See Professor Calvin's remarks on the Devonian System of Iowa, in Rept. 

 Iowa Geol. Surv. 1897, VIII, p. 221; also Professor Schuchert's essay on the Faunal 

 Provinces of the Middle Devonic of America, in Amer. Geol. 1903, 32, pp. 137-162. 



