142 PALEONTOLOGY OF ILLINOIS. 



the slope terminating in a narrow depression, which is bordered by 

 the more or less well-defined marginal fold along the anterior ar- 

 ticular border, that behind merging into the wider depression which 

 is bordered by the narrow, alate lobe of the posterior border. Other 

 dental forms of the mandibles unknown. 



Maxillary posterior teeth trigonal in outline, relatively longer and 

 less strongly arched and inrolled than the opposed mandibular teeth. 

 Antero-lateral border gently curved sigmoidally outward and slightly 

 forward, proportionately low and inbeveled, defined by the narrow, 

 rounded, inbeveled enamel fold, which is separated from the shallow 

 basal rim by a narrow furrow ; the postero-lateral border more ob- 

 liquely converges toward the outer inrolled margin with gentle cur- 

 vature, ^usually strongly inbeveled, the coronal enamel contributing 

 a relatively wide belt inbeveled to the deep, more or less distinctly- 

 channeled basal portion, which meets the inferior surface of the 

 tooth in an obtuse angulation; inner margin very obliquely produced 

 from the slight anterior angle to the base of the coronal prominence, 

 where it is abruptly deflected with a slight concavity in passing the 

 posterior, coronal depression to the sub-acute posterior extremity. 

 The anterior portion of the crown is occupied by a prominent ridge, 

 the more or less rounded, sometimes angular crest, culminating 

 nearest the antero-lateral border, to which it is steeply sloped, and 

 defined by the shallow, depressed belt bordering that side; the op- 

 posite side presenting a wider, gently-convex, or concave slope, de- 

 scending into the posterior depression, which is bounded by the more 

 or less strongly-upraised posterior alation. The character of the 

 anterior teeth associated with the above form has not been deter- 

 mined. 



The superficial characters associated in common with the forms 

 above noticed are subject to variations such as characterize the 

 species they represent. While the normal condition of the crown is 

 that of a smooth, finely and densely punctate surface, to these 

 features certain species add narrow, impressed transverse sulci, 

 which in others assume the magnitude of broad undulations cross- 

 ing the crown parallel to the inner margin of the tooth. There are 

 also, in certain species, more or less distinctly developed longitudi- 

 nal plicae or furrows, which are especially discernible in the less 

 worn surface toward the inner margin. 



The forms above alluded to have already been brought to the 

 attention of naturalists, and their congeneric relationship con- 

 jectured. As early as 1859 Professor Agassiz had recognized the 



