EASTMAN: MYLOSTOMID PALATAL DENTAL PLATES. 269 



the " ferox " type of crushing plate, the same process might have 

 happened quite as readily under favoring mechanical conditions in 

 a related genus, family, and group. That it actually did happen among 

 Ctenodipterines, as evidenced by Heliodus lesleyi, Synthetodus and 

 other forms, has long been known ; and we now contend that the same 

 process took place also in Dinichthys, in such manner as to produce the 

 powerful and elongate compound "shear-tooth." Granting the truth of 

 this proposition, it follows that the vomerine or so-called " premaxillary " 

 teeth of Dinichthys cannot be homologized with either of the two pairs 

 of Mylostomid pavement plates, but are to be regarded as the precise 

 equivalent of the most anterior of the three pairs of upper dental plates 

 (i. e., vomerines) that have been proved to occur in Dinomylostoraa. 

 The far-reaching consequences of this interpretation have been suffi- 

 ciently elaborated in previous numbers of this Bulletin. 



Following is a list of the known genera and species of American 

 Mylostomids : 



MYLOSTOMATIDAE. 



1. Dinomylostoma beecheri Eastm. Portage beds of the Upper Dev- 



onian ; Mt. Morris, Livingston County, New York. 



2. Mylostoma variabile Newberry. Cleveland shale (Upper Devonian) ; 



near Slieffleld, Ohio. 



3. Mylostoma newberryi Eastm. Cleveland shale ; near Sheffield, 



Ohio. 



4. Dinognathus terrelli (Newberry). Cleveland shale; Erie and Lo- 



rain Counties, Ohio. 



5. Diplognathus mirabilis Newberry. Cleveland shale ; Lorain County, 



Ohio. 



