DEVONIAN FISHES OF IOWA 191 



slender shaft of bone, and the dental plate proper ; but the pres- 

 ence of a third element, not distinctly differentiated, is per- 

 haps to be inferred in case Jaekel is correct in his recognition of 

 an articulare in the mandible of Brachydirus. In almost all 

 species of Dinichthys the anterior extremity of the mandible is 

 developed into a powerful piercing beak, usually much facetted 

 by wear. At no great distance behind this the functional mar- 

 gin of the lower dental plate rises again into a prominent projec- 

 tion, shorter and less massive than the one in front, and appear- 

 ing on the inner aspect as a distinct rib-like swelling, nearly vert- 

 ical, and evidently in the nature of a rudimentary tooth. From 

 this point backward along the functional margin, the lower dental 

 plate is compressed into a thin edge, beveled somewhat on the 

 outer face by contact against the opposing palato-pterygoid 

 plate of the upper jaw. In the majority of species, the margins 

 of both upper and lower dental plates are smooth and blade-like ; 

 but a few, including the type, have them denticulated as in Coc- 

 costeus. Vestigial remnants of a primitive Ceratodont-like den- 

 ticulation occur in the lower dental plates along the abrupt de- 

 clivity of their posterior margin; that is to say, in a position 

 corresponding to that in which they are seen in the palato-ptery- 

 goid elements. The Middle Devonian species known as D. lin- 

 colni is peculiar in that a series of tubercles is present along the 

 outer face of the vomerine teeth, thus suggesting that tubercles 

 or denticles were formerly present in the lower dental plates 

 as well, at least in primitive species. 



Dinichthys halmodeus (Clarke). 



(Fig. 27) 



1894. Coccosteus (?) halmodeus J. M. Clarke, Rept. N. Y. State Geol. 1, p. 162, 



pi. 1, and text-figures. 

 1900. Dinichthys halmodeus C. R. Eastman, Journ. Geol. 8, p. 34. 



1906. Dinichthys halmodeus L. Hussakof, Mem. Amer. Mus. Nat. Hist. 9, 



p. 140, text-fig. 22, 24a. 



1907. Dinichthys halmodeus C. R. Eastman, Mem. N. Y. State Mus. 10, 



p. 126, pi. 2, fig. 7; pi. 10, fig. 4; text-fig. 24. 



A primitive species of small size, the headshield having a total 

 length of about 11 cm, and very similar to Coccosteus in the con- 

 figuration cf plates, arrangement of sensory canals, and charac- 



