390 PALEONTOLOGY OF ILLINOIS. 



portions of the convex face and outline of the crest are subject to con- 

 siderable modification, according to the degree of attrition which has 

 taken place while the teeth were in use. By this means, as is the case 

 with all the species of the genus, the convex face rarely exhibits its 

 perfect contour and outline, and in many examples the abrasion has 

 been carried to such an extent as to reduce the vertical diameter of the 

 concave face until it is even less than that of the triturating surface of 

 the convex face. 



Position and locality : The typical example of the above species was 

 obtained by Prof. Worthen from the limestone overlying coal No. 5 1 

 at Belleville, Illinois. A single specimen discovered in the limestone 

 horizon above coal No. 8, at Springfield, Illinois, may prove to be speci- 

 fically identical with the above ; it represents a smaller individual, and 

 the convex face is less abraded, but the base is broken away, although in 

 lateral diameter, as well as in the general outline of the coronal region, 

 it presents most intimate resemblance to the tooth described. 



Genus DACTYLOD US, K and W. 



Dactylodus concavus, St. J. and "WV 



PL XTTT, Fig. 17,18. 



We have, provisionally, recognized the specific distinctness of a form 

 of teeth represented by a few individuals in the collection of Mr. Tan 

 Hornb, which were obtaiued from the same beds which have afforded 

 a magnificent series of Dactylodus princeps, N. and W., with which the 

 present teeth are intimately allied. Indeed the chief distinctions by 

 which they are characterized consist in the deep, angular excavation of 

 the concave face of the crown, the basal border of which is more 

 strongly produced horizontally, the convex face forming a greater angle 

 with the basal area than is observed in the large number of typical 

 examples of D. princeps, the teeth are narrower laterally, and relatively 

 of greater vertical extent, the radicles of the base probably not exceed- 

 ing four, and more strongly defined from the basal area. The basal 

 band in the concave crown-face is equally wide and prominent, consist- 

 ing of three to four strong imbrications, strongly produced and inbev- 

 eled to the base. The convex face is more or less abraded, and granulo- 

 punctate, the punctffi becoming vertically elongated near the crest which 

 is obtusely angular, culminating in an eccentric, rounded apex; concave 

 face smooth, or roughened by the appearance of the minute tubular 

 structure. The outline of the basal margin in the convex crown-face is 

 not dissimilar to that of D. princeps, although it is principally more 

 strongly arched downward in the middle, and forms a much less obtuse 

 angle with the basal area. In size the teeth are smaller than the major- 



