164 PALEONTOLOGY OF ILLINOIS. 



or slightly produced beyond the coronal limits posteriorly. Anterior 

 ridge occupying more than half the transverse diameter of the cor- 

 onal surface, transversely arched, abruptly sloping in front to the 

 anterior articular border, more gently declining into the posterior de_ 

 pression from which the suddenly-upraised, narrow alate lobe is de- 

 fined by a distinct angulation. Surface minutely and closely punc- 

 tate, and further marked by more or less distinct transverse undu- 

 lations conforming to the inner margin, usually obsolete over the 

 greater extent of the coronal region. A specimen of ordinary size 

 shows a length along the coronal ridge of about 28 mm; transverse 

 diameter at the inner margin, 11 mm. 



Posterior tooth of the mandible proportionately agreeing in size 

 with the opposed maxillary teeth, trapezoidal in general outline, of 

 great transverse diameter at the inner margin as compared to the 

 length along the antero-lateral border, strongly inrolled and arched 

 longitudinally. Antero-lateral border obliquely produced outward and 

 forward from the obtuse inner angle, enamel fold forming a narrow, 

 rounded belt inbeveled to the slightly-channeled basal rim, which it 

 equals in depth; inner margin very obliquely produced backward 

 from the anterior angle, broadly arched round the base of the cor- 

 onal prominence with a slight concavity in passing the posterior 

 coronal depression into the moderately produced, acutely rounded 

 posterior extremity, interiorly inbeveled ; postero-lateral border very 

 rapidly converging toward the outer inrolled margin, gently curved 

 and nearly parallel with the anterior half of the inner margin, the 

 narrow, rounded enamel fold inbeveled and denned by a slight sulcus 

 from the basal portion, which forms a shallow rim projecting down- 

 ward and outward well beyond the coronal border, in worn ex- 

 amples inbeveled. Crown traversed nearly centrally by a rather 

 prominent regularly transversely arched ridge, the broad slope de- 

 scending to the antero-lateral border, somewhat depressed, more 

 abruptly sloping into the posterior depression, beyond which rises 

 the moderately upraised alate expansion of the postero-lateral bor- 

 der. Surface closely and regularly punctate, mature individuals, 

 especially those in which the exterior layers have been exfoliated, 

 showing more or less distinct crowded rugse parallel with the inner 

 margin of the tooth. In both of the above described forms a tend- 

 ency to compression or elongation of the punctae is observable, in 

 the transverse furrows presenting a very delicate tracery in some 

 specimens. A specimen of ordinary size measures across the inner 

 margin 18 mm. ; length of autero-lateral border to the inrolled outer 

 margin, 8 mm, 



