No. 1. — Structure and Relations of Mylostoma. By C. K. Eastman. 



It is proposed in the present communication to point out the inti- 

 mate structural resemblance between Mylostoma and Dinichthys, and, 

 taking these forms as typical examples of Arthrodires, to compare their 

 general organization with that of Neoceratodus and other Dipnoan 

 fishes. Evidence is presented for associating Arthrodires with Dip- 

 neusti, and their relations to fossil and recent members of the subclass 

 are considered. A summary is also given of the leading facts in the 

 evolutionary history of Dipnoans since their first appearance in the 

 Lower Devonian until their decadence bordering upon extinction in 

 the modern fauna. 



One of the chief contentions of the present paper is that which 

 relates to the systematic position of Arthrodires ; and as scarcely any 

 two modern writers are agreed upon this matter, it is instructive to 

 review the more prevalent theories concerning the relations of these 

 extinct forms to other fishes. The " family Placodermi " of M'Coy 

 was instituted in 1848 for the reception of Coccosteus, Pterichthys, and 

 Asterolepis, and for more than forty years these genera and their allies 

 were considered to form a natural group of Ganoidei. Elevated by 

 subsequent writers to ordinal and even higher rank, it remained for 

 Cope, in 1889, to recognize the heterogeneous nature of this assem- 

 blage, and to initiate its disruption. He first proposed the removal of 

 Asterolepis from the class of Pisces altogether, and at the same time 

 referred Coccosteans provisionally to the Crossopterygii. (Amer. Nat., 

 1889, 32, p. 856). Shortly afterwards, however, following Smith 

 Woodward's suggestion, the several families of Coccosteus-like fishes 

 were grouped, under Woodward's new term of Arthrodira, in a separate 

 order of Dipnoans. 1 This arrangement obviously implied, though it 

 had not as yet been demonstrated, that the Arthrodiran skull was truly 

 autostylic, and that a secondary upper jaw was not developed. One 

 of the chief reasons which influenced the novel association of Arthro- 



1 Cope, E. D. Syllabus of lectures on geology and paleontology. Philadelphia, 

 1891, p. 14. 



VOL. L. — No. 1 1 



