204 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



Worth., I'sefliodus reticulatus {-pars) St. J. and Worth., Helodus conicidus Nuwb. 

 and Worth., H. triangularis, and H. acutus Davis. To these must also be added 

 the symmetrical tooth described below as Helodus incisus, sp. nov. 



It has been shown by Traquair that the teeth named Helodus planus by 

 Agassiz are certainly referable to Psephodusmacjnus, and Smith Woodward sup- 

 poses that "most of the teeth from the Bristol Bonebed, named Helodus laevis- 

 simus, doubtless pertain to PsepJiodus laevissimus." The other teeth assigned 

 to Helodus, however, are regarded by the same author as " probably common 

 to one or more genera or species, and it is thus convenient, upon present evi- 

 dence, to retain their provisional determinations." For the same reason, also, 

 it is convenient to retain in a provisional sense most of the species which have 

 been described under the name of Chomatodus. 



Choinatodus inconstans St. John and VVorthen, 



(Plate 4, Figs. 33-34.) 



1875. Chomatodus inconstans St. John and Worthon, Pal. Illinois, Vol. VI., p. 360, 



PI. X., Figs. 5-14. 

 1875. Chomatodus varsouviensis St. John and Wortlien, Il/id., j). 363, PI. X., Figs. 



1-4. 

 1875. Chvmatodus chesterensis St. John and Wortlien, Ibid., p. 3G3, PI. X., Figs. 



15-17. 



This species occurs typically in the St. Louis limestone, but it was noted by 

 the Illinois palaeontologists that very similar forms are found also in tliu under- 

 lying Warsaw beds, and in tlie Chester limestone above, to which the names 

 C. varsouviensis and C. chesterensis were given respectively. There can be 

 little impropriety in assigning to the same species teeth of the form shown in 

 Plate 4, Figs. 32-34, which are from the Keokuk Group, thus indicating a con- 

 tinuous existence from this horizon onward throughout tlie Lower Carbonifer- 

 ous. The original of the accompanying figures belongs to the United States 

 National Museum, and a .second specimen is preserved in the Museum of Com- 

 parative Zoology. 



Formation and Locality. — -Keokuk to Chester Groups; Mississippi Valley- 



Helodus incisus, sp. nov. 



(Plate 5, Fig. 54.) 



Type. — Isolated tootli ; Museum of Comparative Zoology. 



Teeth small, bilaterally symmetrical, more or less triangular in cross-section, 

 the crown rising abruptly into a slightly recurved median eminence. Coronal 

 surface uniformly smooth ; posterior face strongly convex, anterior face very 

 gently arched or almost plane, with a large ^-shaped incision; faint ridges 

 extend along the borders of the cavity on either side, and a thipd extends ver- 



