DEVONIC FISHES OF THE NEW YORK FORMATIONS 1 49 



Formation and locality. Genesee of Eighteen Mile creek, N. Y., and 

 Lexington, Ky. ; Portage of Seneca co. N. Y. 



Family jviylosxoma.tida.b 



Head shield and abdominal armor essentially as in the preceding family, 



but with dentition adapted for crushing instead of cutting. Upper dentition 



consisting of two pairs of Ceratodontlike palato-pterygoid dental plates, 



with nondenticulate margins. Vomerine teeth present in at least one genus. 



Genus myl,ostoma Newberry 



Distinguishable from Dinichthys only by characters of the dentition. 

 Oral surface of lower dental plates broad, more or less flattened, and 

 bearing either a rounded boss or V-shaped eminence close to the inner 

 margin, which plays into a corresponding depression of the upper pair. 

 Vomerine teeth as yet unrecognized. 



Our knowledge of Mylostoma is confined at present to three species, 

 all from the Cleveland shale of Ohio. Of these M. var labile New- 

 berry, which is typical of the genus, has become comparatively well known 

 within the last few years, thanks especially to the elaborate researches of 

 Dr Bashford Dean. On the other hand, no additional material illustrating 

 the characters of M. terrelli has come to light since the discovery of 

 the unique type described by Newberry, now the property of the Museum 

 of Comparative Zoology. One of the interesting points established by 

 Dean's study of the type species is the close agreement between it and 

 Dinichthys in all essential respects save for the dentition ; and as regards 

 this latter feature, the same difference is to be noted as exists between 

 Rhynchodus and Palaeomylus amongst Chimaeroids, or between Protopterus 

 and Neoceratodus amongst modern Dipnoans. Parallel modifications of 

 this nature, occurring as they do in diverse groups, are doubtless to be cor- 

 related with similar food habits. Amongst Chimaeroids, for instance, 

 certain genera are shown by their development of tritoral dental plates to 

 have subsisted on hard-shelled prey, such as moUusks, echinoderms and the 

 like ; whereas others, as indicated by their sharp cutting blades, were 



