DEVONIAN FISHES OF IOWA 131 



responding to vomerine teeth or "premaxillaries" as they are 

 sometimes called; and (3) a system of dorsal and ventral armor- 

 ing in the abdominal region, the former articulating with the 

 headshield by a movable joint. None of these structures are 

 found in association with Ptyctodont remains, however, even 

 under the most favorable conditions of preservation. Jaekel, for 

 instance, does not mention an ossified headshield in "Rham- 

 phodus," but notes explicitly that the cranial roof is "composed 

 of calcified cartilage", as is the invariable rule among Elas- 

 mobranchs.* Moreover, there is considerable reason to sup- 

 pose that Ptyctodonts were armed with dorsal fin-spines of the 

 forms known as Heteracanthus, Phlyctaenacanthus, etc., the 

 like of which is unknown among Arthrodires. Problematical as 

 may be the nature of Ptyctodonts, their interpretation as Elas- 

 mobranchs still remains the most satisfactory that has been 

 proposed. 



Formation and locality. Middle Devonian; Eifel District, 

 Rhenish Prussia. Upper Devonian ; Wildungen, Waldeck. 



Bhynchodus pertenuis Eastman. 

 (Text-figure 19) 



1904. Bhynchodus pertenuis C. R. Eastman, Amer. Nat. 38, p. 297, text- 

 fig. 2. 



1906. Bhynchodus pertenuis L. Hussakof, Mem. Amer. Mus. Nat. Hist. 3, 



p. 113, text-fig. 7. 



1907. Bhynchodus pertenuis C. R. Eastman, Mem. N. Y. State Mus. 10, p. 



69, pi. 2, fig. 5. 



Dental plate narrow and elongate, with sharp and extended 

 sectorial margin and knifeblade-like cross- section; anterior beak 

 prominent, no descending symphysial spiniform process, exter- 

 nal surface smooth. 



The unique dental plate upon which this species is founded was 

 obtained from the Chemung of Franklin, New York, and is now 

 preserved in the Albany Museum. The general outlines and 

 proportions of this plate differ from those of all other species, 

 and the absence of a spiniform anterior process is a character 

 affiliating it with the type of the genus, R. secans, and at the 



*Loc. cit. 1901, p. 183. 



