250 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.52. 



Among the arthrodiran plates in the United States National Mu- 

 seum collection from the same liorizon and locality are several that 

 may be provisionally referred to this species, but owing to failure of 

 preservation of the superficial ornament, a positive determination of 

 the species is impossible. There is a tolerably complete antero- 



FlQ. 4.— DiNICHTHYS HALMODEUS (CLARKE). MaKCELLUS SHALE (ERL4.N), LlVONlA SALT SHAFT, NEW 



York. Restoration of HEAD-smELDX?. Lettering as in Fig. 3. 



ventro-lateral plate in the collection, of relatively small size, and 

 also a very good example of the dorsomedian, which has not been 

 previously figured for this species. It is shown of sUghtly less than 

 the natural size in plate 2, figure 1. 



Formation and locality. — Chemung group (Upper Devonian) ; 

 Warren County, Pennsylvania. 



Genus MYLOSTOMA Newberry. 



The arrangement of the grinding elements constituting the upper 

 dentition of the best-known species of this genus, M. variahile, has 

 been the subject of much discussion during recent years, chiefly by 

 American writers. Up to the present time no evidence has been 

 found of the occurrence of a pair of vomerine teeth in this genus cor- 

 responding to the so-caUed ' * premaxiUary " teeth (in reaUty vomerine) 

 of Dinichtliys; but that a pair of such elements was developed is ren- 

 dered at least a plausible supposition from analogy with Dinomylo- 

 stoma, from the Portage shale of western New York. 



Whereas in former years the problem of orientation of palatal 

 grinding plates depended upon more or less theoretical considerations, 

 we are now fortunately provided with evidence of the most positive 



