148 PALEONTOLOGY OF ILLINOIS. 



DELTODUS TKILOBUS, St. J. and W. 



Mandibular median tooth of medium size, wedge-shaped in outline, 

 moderately arched and rather strongly inrolled. Postero-lateral or 

 oblique border converging toward the outer extremity at an angle 

 of about 15 with the opposite border, slightly sigmoidally curved 

 from the sub-acute inner angle to point of inrollment, basal portion 

 of moderate depth, rather deeply and angularly channeled, the in- 

 ferior rim produced downward and slightly outward beyond the lim- 

 its of the enamel fold, which latter forms a wide belt abruptly 

 beveled above, with an inbeveled narrow strip below; antero-lateral 

 border proportionately short, or hardly more than half the length of 

 the opposite articular border, the rounded anterior coronal lobe in- 

 beveled to the relatively shallow-channeled, inbeveled ( ?) basal rim ; 

 inner margin very obliquely produced backward from the very obtuse 

 anterior angle to the broadly rounded base of the principal coronal 

 ridge, thence to the posterior angle, making nearly a right angle to 

 its anterior course, with a slight concavity in the region of the pos- 

 terior depression. The principal ridge occupies half or more of the 

 transverse diameter of the coronal region, rising into a high, rounded 

 crest, the anterior slope steep and terminating in a narrow depres- 

 sion outwardly denned by the rather prominent, rounded lobe trav- 

 ersing the antero-lateral border; the opposite slope is very abrupt, 

 with slight transverse concavity, merging into the rather deep pos- 

 terior depression, which is bordered by the prominent alate lobe of 

 the postero-lateral border. Coronal surface smooth, the inner margin 

 preserving the thin enamel coating, the worn surfaces exhibiting fine, 

 closely arranged puncta3. Breadth of tooth at inner margin about 

 10 mm; length along the oblique border to point of inrollment 14 

 mm. 



The above description is based upon a single nearly perfect tooth, 

 the antero-lateral border being slightly distorted by pressure. Com- 

 pared with allied species, it is readily distinguished from the mandi- 

 bular median form of the Keokuk and Upper Burlington species, 

 Deltodus occidentalis (Liedy, sp.) and D. spatulatus, N. and W., by the 

 distinct lobed condition of the anterior border; also, the alate lobe 

 is more prominent than in D, spatulatus, and less expanded than 

 occurs in D. occidentalis. 



