110 PALAEONTOLOGY OF ILLINOIS. 



enamel fold as observed in the opposite border; inner margin most 

 strongly arched backward in passing the base of the prominent 

 median lobe of the crown, with deep, broad sinus or deflection in 

 the region of the posterior alation, in general presenting a moderate 

 sigmoidal curvature, inferior edge usually inbeveled, though origin- 

 ally, or when entire, continued downward in a nearly vertical basal 

 wall; outline of the outer inrolled margin the reverse of that last 

 described. Coronal region presenting a principal or median ridge 

 culminating near the middle of the crown, the anterior slope mode- 

 rate and uniform to the angulation or slight depression defining the 

 very narrow belt skirting the antero-lateral border; the posterior 

 slope more abruptly descends into the broad depression in that 

 flank, whence arises the prominent posterior alation. Coronal sur- 

 face further marked by subordinate revolving ridges and furrows 

 variable in disposition and distinctness of definition, also ornamented 

 by delicate transverse rugae or reversed imbrications disposed in 

 more or less parallel order conforming to some degree with the 

 direction of the miner margin, though subject to interruption by 

 implantation and bifurcation, also obsolete or variable in occurrence 

 in species and individuals ; otherwise the surface presents the punc- 

 tate structure common to allied genera. Inferior surface, enveloped 

 in the dense homogeneous layer, smooth or faintly striated longi- 

 tudinally, repeating in a less marked degree the coronal contour. 



Median teeth of mandibles relatively narrow, trapezoidal in out- 

 line, arched and strongly inrolled longitudinally; postero-lateral 

 border conforming to the articular border of the posterior form and 

 constituting the longer side of the tooth, antero-lateral border grad- 

 ually converging toward the outer extremity, both borders showing the 

 channeled basal portion bordered above by the narrow inbeveled 

 enamel fold, inner margin obliquely rounded from the posterior to 

 the anterior obtuse angle. Coronal surface occupied by a more or 

 less prominent ridge situated nearest the postero-lateral border, on 

 which side the declivity is abrupt, and defined by a narrow subor- 

 dinate lobe immediately along the border, opposite side more grad- 

 ually sloping towards the antero-lateral border, which is similarly 

 margined by a narrow ridge with intervening shallow furrow. Sur- 

 face ornamentation the same as noted in connection with the pos- 

 terior terminal form. 



Maxillary posterior teeth subquadrilateral in general outline, mod- 

 erately arched and strongly inrolled along the outer margin. Antero- 

 lateral border slightly oblique to the general course of the inner 



