176 PALEONTOLOGY OF ILLINOIS. 



alation. Antero-lateral border slightly curved sigmoidally, present- 

 ing an abrupt declivity from the angularly rounded upper edge, 

 beneath which the coronal enamel forms a well denned belt, sharply 

 limited by the basal portion, which latter projects downward, form- 

 ing a narrow rim along the inferior edge; postero-lateral border 

 rapidly diverging from the outer extremity, terminating in the more 

 or less produced posterior angle, the upper edge enveloped in the 

 narrow enamel fold, which is distinctly defined from the inferior 

 basal border ; inner margin making a moderate sigrnoidal curvature, 

 varying in obliquity to the antero-lateral border, with which it may 

 form a right-angle. 



Maxillary posterior teeth characterized by the relatively narrow 

 transverse diameter at the inner margin, and consequent wedge- 

 shaped outline. General contour of the coronal region resembling 

 that of the opposed mandibular teeth, the principal prominence ex- 

 tending along the antero-lateral border, slightly convex or plane 

 transversely, more or less steeply sloping into the longitudinal 

 median depression, which is bordered on the opposite side by the 

 upraised narrow wing expansion terminating in the posterior angle ; 

 antero-lateral border gently curved sigmoidally, more or less abruptly, 

 sometimes vertically, sloping from the angular superior edge at the 

 junction with the crown, the enamel forming a belt well denned 

 from the basal border which terminates inferiorly in a thin rim ; 

 the opposite side gradually converges from the posterior angle toward 

 the outer extremity, the upper edge enveloped in a distinct, narrow 

 enamel fold, and denned by a slight channel from the equally shal- 

 low basal border; inner margin oblique in one or other direction to 

 antero-lateral border, with which it also sometimes forms a right- 

 angle. 



The triturating surface in the typical species of both the above 

 described forms presents a fine transverse striated appearance, pro- 

 duced by the arrangement of the punctse in more or less parallel 

 rows, conforming in direction to the course of the inner margin. 

 However, the latter feature is not observed as a persistent one over 

 the entire extent of the coronal region, even in the typical species, 

 while in others it is but obscurely apparent, and in some species 

 presumably otherwise agreeing with the typical forms, the surface 

 presents no vestige of the transverse striation, on the contrary ex- 

 hibiting the usual dense punctate structure common to the gener- 

 ality of the Cochliodonts. 



