204 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Worth., Psephodus reticulatus ( pars) St. J. and Worth., Helodus coniculus Newb. 
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 Psephodus magnus, and Smith Woodward sup- 
poses that “most of the teeth from the Bristol Bonebed, named Helodus laevis- 
semus, doubtless pertain to Psephodus 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. 
Chomatodus inconstans Sr. Joun and WorrHeEn. 
(Plate 4, Figs. 32-34.) 
1875. Chomatodus inconstans St. John and Worthen, Pal. Illinois, Vol. VI., p. 360, 
Pl. X., Figs. 5-14. . 
1875. Chomatodus varsouviensis St. John and Worthen, Jbid., p. 863, Pl. X., Figs. 
1-4, ' 
1875. Chomatodus chesterensis St. John and Worthen, /bid., p. 368, Pl. 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 the under- 
lying Warsaw beds, and in the 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 the 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 tooth; 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 A-shaped incision; faint ridges 
extend along the borders of the cavity on either side, and a third extends ver- 
