EASTMAN: STRUCTURE AND RELATIONS OF MYLOSTOMA. 3 



the Birkeniidae. All of these forms, or if the expression be permitted, 

 Placoderms in the Jaekelian sense, were considered to be true fishes. 1 

 It was further maintained by the same author on more than one occa- 

 sion that Coccosteans are ancestral to Chimaeroids, an opinion that may- 

 be compared with Newberry's idea that Protopterus and Lepidosiren are 

 modern survivals of Dinichthys. 2 Newberry and Jaekel thus stand 

 alone in the recognition of any descendants of Arthrodires. 



We may now pass rapidly in review the minor fluctuations of opinion 

 that are apparent during the last few years. Dr. 0. P. Hay, in his 

 Catalogue of fossil Vertebrata of North America (1902), employs the 

 term Placodermi for both Arthrodires and Asterolepids, placing them 

 in the same subclass as Dipnoans. Arthrodires and Ostracophores are 

 awarded each the rank of a separate subclass in the English edition of 

 von Zittel's Textbook of paleontology, the author having discountenanced 

 an association between Coccosteans and Dipnoans. In a remarkable 

 paper by C. T. Regan, published in 1904, the Placodermi are re- 

 established so as to include the Coccosteidae, Asterolepidae, and Cepha- 

 laspidae, all being united in a single order of Teleostomes. During the 

 same year Professor Bridge expressed the view, in the volume on Fishes 

 in the Cambridge natural history, that Coccosteans are "a highly 

 specialized race of primitive Teleostomi," and compared their cranial 

 roof-plates with those of typical bony fishes. Both in this work and in 

 an elaborate monograph on the skull in modern Dipnoans, this author 

 dissents emphatically from the opinion that Arthrodires and lung-fishes 

 are at all closely related. Thus, in the volume on Fishes, at page 537, 

 we read as follows : — 



" The Arthrodira have been regarded as armoured Dipneusti, a view which is 

 mainly based on their supposed autostylism and the nature of the dentition. But 

 this autostylism has yet to be verified, and, if proved, the possibility that it may 

 be a secondary feature, associated with the evolution of a peculiar dentition, must 

 not be forgotten. Much more may be said for their claim to be regarded as a 

 highly specialized race of primitive Teleostomi. Besides a well-developed lower 

 jaw, bones comparable to the elements of a secondary upper jaw are known, and 

 in a general way the disposition of the cranial roofing bones, and the arrange- 

 ment of the endoskeletal elements of the pelvic fins, tend to conform to the normal 

 Teleostome type. In fact, Dr. Traquair has expressed the opinion that the 

 Arthrodira are Teleostomi and Actinopterygii." 3 



1 Jaekel, O. Ueber die Organisation und systematische Stellung der Astero- 

 lepiden. Zeit. Deutsch. Geol. Gesell., 1903, 55, p. 55. 



2 Newberry, J. S. Rept. Geol. Surv. Ohio. Paleont., 1875, 2, p. 15. 



3 In his latest reference to this subject, however, they are stated by Traquair 

 to be of uncertain subclass. Cf. Trans. Roy. Soc. Edinb., 1903, 40, p. 732. 



