DEVONIAN FISHES OF IOWA 215 



stood as correlated with difference in position in the mouth; the 

 thin, flat, multico state plates belonging to the upper, and the 

 heavier, more convex, fewer ridged plates to the inferior 

 dentition. The species referred to is D. mordax, which appears 

 to be numerically the most abundant of all Dipterines repre- 

 sented in the State Quarry beds, though seldom perfectly pre- 

 served. Even fragmentary plates, however, are as a rule easily 

 recognized by their exceedingly coarse tuberculation, paucity of 

 costae in the adult, and subtriangular form — flattened, or even 

 plane or concave in the upper, more or less arched in the lower. 

 Owing to their tenuity, and consequent liability to destruction, 

 the upper or "flabelliform" type of plates are less numerous 

 than the lower, and their attenuated external margins are often 

 imperfect or broken away. 



As regards general manifestation and relative profusion, D. 

 mordax occupies much the same position in the Iowa Upper De- 

 vonian as does D. nelsoni in the Chemung of the Appalachian 

 district. The inference is not far to seek that not only the Che- 

 mung species, but also those from the eastern Catskill — which 

 are alike coarsely tuberculated — are derivatives or modifications 

 of an immigrant western type, which existed plentifully in the 

 Upper Devonian of Iowa, and w nose advent in the Middle De- 

 vonian of the same State is heralded by D. uddeni at the base of 

 the Cedar Valley limestone. Noteworthy is the fact that no 

 indications of Ctenodipterines have been discovered in the Mid- 

 dle Devonian east of the Mississippi Valley region. The signifi- 

 cance of this dearth of remains is twofold. First, it proves that 

 no immigrants entered the Appalachian basin from the eastward 

 during the early or middle part of the Devonian, and probably 

 not at all, as the Chemung invasion (witness the resemblance 

 between D. nelsoni and D. mordax) in all likelihood came in from 

 the west. Secondly, the evidence furnished by the facts of 

 Dipterine distribution is consonant with the prevailing theory 

 that intercommunication between the Dakotan and Ohioan seas 

 was not effected until towards the close of Hamilton time (cf. 

 Plate XV). 



In the course of the foregoing remarks we have confined our 

 attention purposely to the two leading species of Upper De- 



