136 



IOWA GEOLOGICAL SURVEY 



margin projecting downward as a long spiniform process. An- 

 terior margin of upper dental plate regularly arched, not pro- 

 duced into a beak or spinous process, and showing on the outer 

 face marks of contact with the opposing dentition. Inner sur- 

 face of both upper and lower dental plates with a roughened, 

 deeply incised triangular symphysial facette. 



Fig. 20. 



Fig. 21. 



Fig. 22. 



Fig. 20, 21.— Ptyetodus ferox Eastm. Hamilton; Milwaukee, Wis. To the left is shown 

 an immature lower dental plate with well preserved descending process at the front margin; 

 to the right an older and much worn lower dental plate from which the descending process 

 and a portion of the inferior border have been broken away. Both plates have distinctly 

 marked tritoral areas with obliquely directed punctae, x 1-1. 



Fig. 22.- Ptyetodus ferox Eastm. Hamilton; Milwaukee, Wis. Left upper dental plate 

 showing facet due to wear against the lower dentition, x t-1. 



The fact that the smaller and younger plates belonging to this 

 species have a decidedly Rhynchodus-like aspect (text-fig. 20), 

 indicates that the functional margin in immature teeth was sim- 

 ply trenchant, the tritors not being developed until a compara- 

 tively late period. Rhynchodus, therefore, represents a more 

 primitive stage than either Ptyetodus or Paloeomylus in the 



