104 PALEONTOLOGY OF ILLINOIS. 



Teeth supposed to represent the median form of the mandibles, 

 small, trapezoidal in outline, strongly arched from within outward. 

 Postero-lateral (?) border nearly straight, with slight obliquity in 

 its outward and slightly forward course, basal portion evidently 

 shallow, channeled, and denned above by the relatively strong, 

 rounded enamel fold; antero-lateral or oblique border converging 

 from the sub-acute inner angle to point of inrollment at an angle 

 of about 20 with the opposite border, the strong, rounded enamel 

 fold projecting beyond the comparatively shallow, channeled basal 

 rim; inner margin somewhat abruptly rounded from the obtuse 

 posterior angle, thence nearly straight to the anterior angle, usually 

 inbeveled inferiorly. Coronal surface showing a wide, gently depressed 

 acclivity very gradually rising from the antero-lateral border culmi- 

 nating in a rounded ridge, the posterior slope of which abruptly 

 descends to the postero-lateral border fold, from which it is defined 

 by a narrow parallel depression. The latter feature may become 

 obsolete in worn specimens indeed the whole outer half of the 

 coronal surface usually bears evidences of excessive wear from use. 

 The inner margin of the crown preserves the enamel coating, through 

 which the somewhat irregular orifices of the tubular structure are 

 distinctly discernible, worn surfaces showing a fine pitted structure 

 with the longitudinally elongate punctse exactly as remarked in 

 the before mentioned forms. A medium sized tooth measures in 

 greatest diameter between the inner angles 7.5 mm ; length along 

 postero-lateral border to point of inrollment nearly 4 mm. 



In assigning the last above described form to Stenopterodus, we 

 have been guided chiefly by the marked resemblance of the surface 

 punctation to that observed in the posterior forms of the upper and 

 lower jaws with which it is associated in the same deposits. But 

 while the above forms are represented by several individuals each, 

 the collections contain only three examples of the present form, and 

 two of these are proportionately much smaller than any of the 

 examples of the mandibular posterior teeth with which it is sup- 

 posed this form was specifically associated; but Mr. Springer's col- 

 lection contains a single imperfect specimen of a larger individual 

 which corresponds in relative size to the ordinary dimensions of the 

 mandibular posterior teeth. 



In general outline and coronal contour these teeth bear some 

 resemblances in common with the forms provisionally identified with 



