416 PALEONTOLOGY OF ILLINOIS. 



depressed, and probably broadly rounded below, and defined by imbri- 

 cating folds. Boot unknown. Coronal surfaces smooth or punctate. 



The present genus is recognized in a single species from the upper 

 division of the Coal Measures. Unfortunately none of the specimens 

 are entire along the basal margin, affording no clue to the form of the 

 root and outline of the basal margin of the concave crown-face. The 

 root is probably small, and from the general similarity in contour of 

 the other parts to Peltodus and Fissodus, it seems not improbable that 

 it may prove to be" similar in shape to the root in those genera. The 

 superficial markings are also similar to those observed iu the above 

 named genera, the distinctive generic characters being chiefly presented 

 in the eccentrically lobed crest and the extreme downward prolongation 

 of the lateral extremities of the coronal fold in the convex face. 



The association of the individuals of the sole representative species 

 known at the present time with the teeth of Petalodus uriguiformis might 

 raise a suspicion of the generic, possibly specific, relations or identity 

 of these forms. But the facts possessed do not sustaiu such a conjec- 

 ture, unless it proves to be a species of most extraordinary variableness, 

 the mutations of which in some instances would be even more remark- 

 able than the characters by which many of the Petalodont genera are 

 distinguished from one another. 



Cholodtjs in^qualis, St. J. and W. 



PL XIII. Kg. 4, 5. 



Teeth of medium size, in general outline subovate or rhomboidal. 

 Basal area forming a gentle convexity uniform with the convex crown- 

 face, from which, in the worn specimens usually met with, it is defined 

 by a low, distinct border, produced by the thickening of the upper rim 

 of the basal area and thus raised slightly above the plane of the crown, 

 but in well preserved teeth, in -which the original external coating of 

 the crown still exists, an exceedingly delicate raised line marks the 

 basal limit of the crown, corresponding to the imbricated folds in allied 

 genera ; the basal line spans the tooth in an irregular direction, more 

 or less oblique from one or other side, slightly arched downward in the 

 middle or quite direct, but on neariug the lateral margins it is suddenly 

 curved downward in the direction of the infero-lateral angles of the 

 tooth. The concave face of the crown presents a gentle concavity, 

 gradually rising into the irregularly cuspidate crest, and slightly 

 rounded into the thickened or compressed lateral margins, 'which are 

 nearly parallel though slightly and irregularly curved, and slightly 

 diverging from the basal angles ; the crest presents an irregular out- 

 line, being divided into two prominent, unequal lobes, which culminate 



