THE DEVONIAN FISHES OF MISSOURI 67 



height of crown 3 mm. The crown narrows gradually from 

 the middle to the ends and is highest in the middle though 

 the elevation is not localized into a cone. The outer ends 

 of the tooth curve slightly backward. The crown is closely 

 and finely punctate. 



Horizon and locality: Near the top of the Devonian 

 shale at Louisiana, Missouri. 



Helodus incipiens sp. nov. 

 (Plate IV, figs. 11-16) 



Types four teeth in Mr. Rowley's collection. Teeth 

 small and simple, gently arched toward the top but without 

 median cone. Sides subparallel. No crenulations or orna- 

 mentations of any kind are present on the crown. One tooth 

 shows transverse corrugations across the base of the root. 

 Root generally missing. These teeth show some variation 

 in shape and size but are generalized and of a type that may 

 have given rise to all of the early Mississippian helodonts. 

 If the shales from which they were collected are Devonian in 

 age these are among the oldest of the pavement toothed sharks 

 known from America. In the Chouteau of Sedalia and Provi- 

 dence, Missouri, three or four species occur that may be dis- 

 tinguished from the Devonian teeth by minor characters only 

 and seem to be direct descendants of the Devonian forms. 



Horizon and locality: Near the top of the Devonian 

 shales at Louisiana, Missouri. 



Helodus clivulus sp. nov. 

 (Plate IV, figs. 1, 2, 5, 6) 



Types two incomplete teeth. These teeth are not far 

 removed from Helodus incipiens but have a low median boss 

 instead of the curving surface of that species. The crown 

 part at the ends is thinner than in that species and the puncta- 

 tion seems to be different, though it is impossible to determine 

 this positively without sectioning the teeth. 



Horizon and locality: Near the top of the Devonian 

 shales at Louisiana, Missouri. Collection of Mr. Rowley. 



