102 PALEONTOLOGY OF ILLINOIS. 



sumed to identify with the mandibular median form of the present 

 genus. The latter teeth are subrhombic or trapezoidal in outline, 

 with a nearly straight postero-lateral border, and longer and more 

 obliquely converging opposite border, defined by the same style of 

 enamel fold and basal rim, characteristic of the previously men- 

 tioned forms; the coronal region presents a broad, nearly plane 

 surface, gradually culminating near the postero-lateral borders in a 

 depressed, obscurely defined ridge, where the slope to the border on 

 the one hand is abrupt, and on the other very gentle, the inner 

 margin gently arched and somewhat rapidly deflected to the obtuse 

 posterior angle. 



The genus as now understood is represented by species beginning 

 in the Upper Burlington limestone, and in each succeeding forma- 

 tion of the Lower Carboniferous period, the latest authentic occur- 

 rence being in the St. Louis limestone. 



STENOPTEEODUS PLANUS, St. J. and W, 



PI. IV, Pig. 9-14. 



Represented by two forms of teeth below medium size, presumably 

 occupying corresponding positions on opposite jaws. 



Posterior teeth of the upper jaw sub-rhomboidal in outline, rather 

 strongly inrolled and arched longitudinally. Antero-lateral border 

 slightly oblique in its forward and outward course, basal rim ap- 

 parently shallow, and defined from the narrow enamel fold by a 

 shallow groove ; postero-lateral border converging at an angle of 

 about 25 with the opposite border, the basal rim somewhat thick- 

 ened and produced beyond the limits of the narrow, inrolled enamel 

 fold, inner margin broadly arched with a slight angulation near the 

 middle in passing the base of the coronal prominence, in front of 

 which a sharp, slight constriction occurs, the inferior edge in beveled 

 in the worn condition; posterior extremity forming a sub-acute 

 angle, the margin making an obtuse angle with the anterior border. 

 Coronal prominence comprising more than half the lateral area of 

 the surface, culminating in a low, rounded crest near the median 

 line, with an abrupt slope to the anterior neck, defined by an in- 

 tervening shallow groove, the posterior broad slope very gently and 

 faintly depressed transversely; the anterior neck somewhat rapidly 

 narrows to the antero-lateral border, is gently convex transversely and 

 marked by faint, longitudinal plicae. The whole outer third or more 

 of the crown surface presents an even, smooth, triturating area, the 

 result of wear, exposing to view the minute, regular punctae, which 



