44 Bulletin American Museum of Natural History. [Vol. XIX, 



Fig. 3T. A nog- 

 VI i u s alius? 

 (Loomis). Same 

 individual as Figs. 

 27-30. X 2; In- 

 ferior grinding 

 plate. 



Fig. 32. A nog- 

 nt iu s alius? 

 (Loomis). Section 

 through bone of 

 Fig. 31. X 2. 



the structure. Fig. 32 shows a small portion magnified two 

 times. The pits and cavities in the bone are shown by the 

 black. It will be observed that the pits pen- 

 etrate nearly the whole thickness of the bony 

 mass. Loomis has described the dense mass 

 which forms the bulk of these triturating plates 

 as "dentine" and "osteodentine." I do not 

 discover the peculiar structure of dentine in 

 any of them. The canals which Dr. Loomis 

 calls "Haversian" appear to be the pits which 

 open on the surface. To me the plates appear 

 to be merely a kind of dense bone, similar to 

 that found on the maxilla and premaxilla out- 

 side of the mouth cavity; and these bones dis- 

 play not very dissimilar pits. Dr. Woodward 

 has mentioned the presence of dentinal tubules 

 in Plcthodus (Ann. and Mag. Nat. Hist., Ser. 

 7, Vol. Ill, p. 355). Unfortunately he has not 

 furnished figures of these microscopical ele- 

 ments, or given us their dimensions. What is 

 the function of these pits, and how they have 

 been produced, the present writer does not 

 attempt to explain. It would be interesting 

 to know how these bony masses increased in 

 thickness. 



Seen from the lower side this bone is orna- 

 mented with fine ridges, which run, for the most part, longi- 

 tudinally and anastomose, but which, in some parts, run in 

 other and often irregular directions. In cross-section these 

 ridges form narrow perpendicular plates, as appears at 

 the bottom of Fig. 32. They appear to be similar to the 

 fine ridges found by Dr. Woodward in similar situations in 

 Plcthodus. 



The convex surface of this plate fits well the concave sur- 

 face of the upper plate. Fig. 7,7, presents a view of the grind- 

 ing surface of a lower dental plate collected for the author 

 in the region of Butte Creek, Kansas, but which is now the 

 property of the American Museum. It is strongly convex 



Fig. 33. A nogmius 

 sp. No. 994. X \. 

 Inferior dental plate. 



