98 PALEONTOLOGY OF ILLINOIS. 



DELTOPTYCHIUS EXPANSUS, St. J. and W. 



PI. V, Fig. 9, 13. 



Posterior teeth of the mandibles, trigonal in general outline, outer 

 extremity strongly inrolled, relatively broadly expanded posteriorly, 

 and broadly arched along the inner margin with sigmoidal curvature 

 in passing to the posterior extremity of the strong wing expansion^ 

 Antero-lateral border slightly curved sigmoidally, moderately deep, 

 the greater portion of the vertical face being covered by the coronal 

 enamel and making a well-defined angulation above; postero-lateral 

 border rapidly converging toward the extremity, basal portion pro- 

 jecting somewhat beyond the narrow and prominent fold limiting 

 the coronal surface, and more or less produced posteriorly. Coronal 

 surface showing a broad, deep posterior depression, which, together 

 with the alate border, constitutes half the lateral diameter of the 

 tooth; median lobe relatively narrow, generally obscurely defined in 

 front from the narrow anterior plane belt with which it is usually 

 merged over the greater extent of the outer surface. Length of a 

 medium-size tooth along antero-lateral border 8 mm. ; breadth at 

 inner margin 8.5 mm. 



Posterior tooth of the upper jaw in outline spatulate, terminating 

 posteriorly in an eccentric sharply rounded spur, antero-lateral bor- 

 der obliquely truncated from inner angle outward and forward. Cor- 

 onal prominence comprising nearly two-thirds the transverse diameter 

 of the" tooth, posterior lobe moderately convex transversely, subor- 

 dinate lobe relatively broad and prominent, anterior area smooth, 

 and narrowed toward the antero-lateral border. Surface minutely 

 and closely punctate, in the same manner observed in the opposed 

 teeth. Greatest transverse diameter of a medium-size tooth about 

 12 mm. ; greatest diagonal breadth across the posterior prominence 

 5 mm. ; length along antero-lateral border 4.5 to 5 mm. 



Of other dental forms occurring with the above described teeth, 

 none have been recognized as probably specifically identical. In 

 superficial coronal characters, as the punctation and appearance of 

 the enamel layer, the two forms above noticed possess most inti- 

 mate characters in common, so that, notwithstanding the fact that 

 all the material in our possession consists of isolated teeth, little 

 doubt is entertained as to their having constituted parts of the 

 dentition of the same species. 



Compared with the Keokuk species, D. Wachsmuthi, the present 

 teeth present well-marked differences which serve to distinguish them 



