DEVONIC FISHES OF THE NEW YORK FORMATIONS 6/ 



Chimaeroid skeletons, however, have hitherto been satisfactorily deter- 

 mined from Paleozoic rocks, save for the possible exception of Menaspis. 

 The problematical remains known as Dictyorhabdis prise us, from 

 the Lower Siluric of Colorado, are very doubtfully of Chimaeroid nature. 

 The most recent discussion of the relations between fossil and living 

 Chimaeroids is that of Dr Bashford Dean, in the publications of the Car- 

 negie Institute of Washington, 1906. Some changes in the classification 

 are proposed by Mr C. Tate Regan in the Proceedings of the Zoological 

 Society of London for the same year. 



Family f'Tyctodonxidae 

 A family at present indefinable, of doubtful ordinal position, known 

 only by remains of the dentition and possible dermal ossifications. A 

 single pair of large, laterally compressed dental plates present in each jaw, 

 united at the symphysis, and either with trenchant oral margin, or develop- 

 ing one or more tritoral areas posteriorly. Rhynchodus undoubtedly rep- 

 resents the most primitive condition of dental plates, the tritors Ptyctodus 

 and Palaeomylus arising subsequently, and indicating a more specialized 



stage. 



Genus rhynchodus Newberry 



Oral margin of dental plates simply trenchant, without tritoral areas. 



Upper and lower dental plates of similar form, except that in some species 



the symphysial margin is produced downwards into a spiniform process. 



Indications of cartilaginous union between each pair of dental plates are 



present on the inner symphysial facets. 



Rhynchodus secans Newberry 

 1873 Rhynchodus secans J. S. Newberry. O. Geol. Sur. Rep't. v. i, pt 2, 



p. 310, pi. 28, fig. i; pi. 29, fig. I, 2 

 1889 Rhynchodus secans J. S. Newberry. U. S. Geol. Sur. Monogr. 16:47, 



pi. 28, fig. 1-3 

 1898 Rhynchodus secans C. R. Eastman, h m. Nat. 32 : 485, 546 

 This species, which is the type of the genus, is not uncommon in the 

 Columbus and Delaware limestones of Ohio and is interesting for having 



