DEVONIC FISHES OF THE NEW YORK FORMATIONS 73 



marks of contact with the opposing dentition. Inner surface of both upper 

 and lower dental plates with a roughened triangular symphysial facet. 



The fact that the smaller and younger plates belonging to this species 

 have a decidedly Rhynchoduslike aspect indicates that the functional mar- 

 gin in immature teeth was simply trenchant, the tritors not being developed 

 until a comparatively late period. Rhynchodus, therefore, represents a 

 more primitive stage than either Ptyctodus or Palaeomylus in the develop- 

 ment of dental plates, its relations to contemporary types being closely par- 

 alleled by Rhinochimaera amongst recent genera.' There is considerable 

 reason for believing that Ptyctodonts were provided with dermal ossifica- 

 tions, and in particular it is difficult to avoid the suspicion that the fin 

 spines known as Heteracanthus or " Gamphacanthus " were borne by mem- 

 bers of this family, possibly by the very species under discussion. The 

 dental plates of P. ferox are accompanied not only by dermal spines, but 

 by tuberculated dermal plates not unlike those associated with Myriacan- 

 thus. One of these is illustrated in plate i, figure 6, of the present 

 memoir. As yet, however, no specimens have been found which demon- 

 strate the supposed specific identity of these various bodies. 



Formation and locality. Hamilton limestone (Erian) ; Milwaukee, Wis. 

 Cedar Valley limestone (Mesodevonic) of Iowa and Illinois. State Quarry 

 beds (Neodevonic) ; Johnson county, Iowa. 



ICHTHYODORULITES 

 Genus onchus Agassiz 

 Spines of small size, laterally compressed ; sides of exserted portion 

 ornamented with smooth or faintly crenulated longitudinal ridges ; no 

 posterior denticles. Represented in this country by two Upper Siluric (O. 

 clintoni and O. pennsylvanicus) and one Devonic species. An 

 undescribed form is also known from the Niagaran of Cumberland, Md. 



' Garman, S. Chimaeroids, especially Rhinochimaera and its Allies. Museum Comp. 

 Zool. Bui. 1904, 41: 246. Dean, B. Chimaeroid Fishes and their Development. Carnegie 

 Inst. Wash. 1906, Pub. 32, p. 126. 



