158 IOWA GEOLOGICAL SURVEY 



pel us to look upon Neoceratodus as an archaic survival of the 

 primal Dipnoan stock, will be discussed under the caption of 

 Arthrodires immediately following. 



Order ARTHRODIRA. 



Dipnoans having a reduced number of dermal bones forming 

 the cranial roof, arranged after essentially the same pattern 

 as in Ceratodonts, and the dentition also paralleling that of 

 modern forms. Dermal armor of abdominal region consisting 

 of large plates, either in simple apposition with the headshield, 

 or more commonly articulated with its posterior border by a 

 pair of movable ginglymoid joints placed dorso-laterally. Col- 

 umn notochordal, but with distinct neural and haemal arches. 

 Tail apparently diphy cereal in the best known forms (Coccos- 

 teus and Dinichthys) ; pectoral fins wanting, and only obscure 

 traces of the pelvic pair observed; pelvic arch represented by 

 a pair of sigmoidal or club-shaped plates, sometimes (Dinich- 

 thys) with an anterior ventral projection. 



The remarkable group of armored Coccosteus-like fishes was 

 originally united with Asterolepids by M'Coy, in 1848, in a 

 single "family Placodermi", and for more than forty years this 

 arrangement was adhered to by writers generally, save for 

 slight changes in the rank assigned to the main divisions. To 

 Professor E. D. Cope belongs the credit of having been the 

 first naturalist to recognize the heterogeneous nature of this 

 assemblage, and to initiate its disruption. In 1889, he proposed 

 the removal of Asterolepids from the class of Fishes altogether, 

 and at the same time referred Coccosteans provisionally to the 

 Crossopterygii, or "fringe-finned ganoids".* Shortly there- 

 after, however, following Smith Woodward's suggestion, the 

 several families of Coccosteus-like fishes were grouped, under 

 Woodward's new name of Arthrodira, in a distinct order of 

 Dipnoans.f This arrangement obviously implied, though it had 

 not at that time been demonstrated, that the Arthrodiran skull 

 was truly autostylic, and that a maxillary arch was not devel- 



* Cope, E. D., Synopsis of the Families of Vertebrata. Amer. Nat. 1889,23, 

 p. 856. 



t Ibid, 1891, 25, p. 647. Also Syllabus of Lectures on Geology and Palaeontol- 

 ogy, p. 14. Phila. 1891. 



