208 PALAEONTOLOGY OF ILLINOIS. 



The enamel belt also distinctly defined and delicately sculptured 

 in vertical rugosities, traces of which are also detected in the belt 

 of the anterior and posterior margins; outer border gently curved, 

 and very gradually diverging from the rounded anterior angle to 

 the posterior angle, which terminates in a somewhat produced 

 spur, the basal portion projecting prominently beyond the border of 

 the crown, which is limited by a narrow enamel fold. The coronal 

 region presents a remarkably uniform surface, in the main gently 

 depressed transversely without well-defined inner prominence, and 

 rather abruptly upraised along the outer border, which forms the 

 most prominent part of the coronal surface. The worn surface ex- 

 hibits minute punctation, and the less worn portions show a delicate 

 reticulate structure, produced by the walls of the vertical prisms 

 enclosing the medullary tubes. Greatest diameter across the poste- 

 rior margin, 53 millimeters ; length along the inner margin, about 30 

 mm. ; breadth across anterior margin, 38 mm. ; thickness of tooth, 

 about 15 millimetres. 



Mandibular form subquadrangular in outline, moderately arched 

 in both directions. Inner border marked by a slight angulation de- 

 fining the anterior and posterior articular facets which are appar- 

 ently nearly equal in extent, coronal enamel well defined along the 

 deep, slightly inbeveled basal portion ; exterior border gently arched, 

 very gradually converging toward the obtuse or sharply rounded 

 anterior angle, the basal portion projecting more or less beyond, 

 and well defined from the rounded coronal border ; anterior and pos- 

 terior margins making the same slight sigmoidal curvature, with a 

 shallow concavity in front and a corresponding broad convexity 

 behind from the inner angles, the basal portion all around more or 

 less distinctly marked by vertical rugosities. The greater area of 

 the coronal region is occupied by the broad low convexity of the 

 inner prominence, which is rounded to the inner border, and on the 

 other hand slopes into the narrow shallow depression bordered by 

 the narrow fold along the outer border ; in front, the coronal prom- 

 inence presents a lunate, slightly depressed area, extending back 

 from the anterior margin a greater or less distance, evidence of wear 

 while in use. The surface presents, apparently, the same character 

 of punctation observed in the above noticed form. Greatest trans- 

 verse diameter across the posterior margin, 35 mm. ; length along 

 inner border, 26 mm. ; breadth at the anterior margin, 28 mm. 



The present species, represented by the two above mentioned 

 forms, is known from half a dozen individuals in a more or less 



