DEVONIAN FISHES OF IOWA 179 



sile to about the same degree as the symphysial beaks of the 

 lower jaw, against which they closed. Their posterior face is 

 smooth and slightly hollowed, thus indicating that they were in 

 direct contact with the anterior pair of palato-pterygoid plates, 

 which are missing in the original example. The posterior pair, 

 however, is admirably preserved, and on being brought into ad- 

 justment with the opposing lower dentition, it is a comparatively 

 easy matter to restore the outlines of the pair immediately pre- 

 ceding. The external margin of the anterior pair must have 

 been parallel to that of the functional surface of the lower dental 

 plate ; and as the upper elements were probably in contact with 

 each other in the median line, the inner margin was rectilinear, 

 as in Mylostoma (cf. text -fig. 25, page 181). 



The dorsomedian plate, with well developed posterior process, 

 agrees closely in form and proportions with that of Dinichthys 

 mtermedius. The antero-ventro-laterals have 'also approxi- 

 mately the same form, but are nearly one-fifth smaller than the 

 corresponding plates in the tolerably complete example of Mylos- 

 toma variabile figured by Dean.* In that author's restoration of 

 the ventral armor, however, these plates have been interchanged 

 with the postero-ventro-laterals, as is evident from an inspection 

 of their respective centers of ossification, and direction of vas- 

 cular canals. 



Formation and locality. Cashaqua shale (Portage) ; Mt. Mor- 

 ris, Livingston county, New York. There is also indistinct evi- 

 dence either of this or some other Mylostomid in the black 

 Naples shale (Portage) at Sturgeon Point, on the south shore 

 of Lake Erie near Buffalo; and an undescribed species, appar- 

 ently of this genus, is thought by Dr. L. Hussakof to be indicated 

 by dental plates from the New Albany (or Genesee) Black shale 

 near Louisville, Kentucky. 



Genus MYLOSTOMA Newberry. 



Distinguishable from Dinichthys only by characters of the 

 dentition. Oral surface of lower dental plates broad, more or 

 less flattened, and bearing a rounded boss or V-shaped prorn- 



*Deati, B., Palseontological Notes: On the Characters of Mylostoma Newberry. 

 Mem. N. Y. Acad. Sci. 1901, 2, p. 108, pi. 7. 



