392 PALEONTOLOGY OP ILLINOIS. 



Dactylodtjs excavatus, St. J. and W. 



PI. SIH, Kg. 16. 



Teeth of small size, moderately strong. Crown thick below, culmina- 

 ting in a sharp, broadly arched crest ; concave face oval in outline, the 

 crest and base inequally arched, rather deeply excavated towards the 

 base, which is defined by a rather strong basal band composed of three 

 or four folds, strongly arched downward in the middle and regularly 

 and gently curved at the extremities, strongly produced along the exte- 

 rior margin and inbeveled, superior coronal surface nearly plane late- 

 rally, smooth and polished ; convex crown-face irregularly lenticular in 

 outline, sharply rounded at the extremities, very gently arched late- 

 rally, nearly plane vertically, crest line moderately and regularly arched, 

 basal border gently and irregularly arched downward, and broadly 

 curved to one or other extremity, defined by a narrow basal fold contin- 

 uous with that of the concave face, but obscure from wear, the entire 

 surface showing evidences of attrition. The basal area forms a narrow, 

 plane surface nearly at right angles to the convex crown-face, merging 

 into the thick, massive radicles below, of which there are indications of 

 three, possibly four, which are produced in a direction corresponding to 

 the plane of the concave crown-face, probably equaling if not exceed- 

 ing in depth the bight of the crown, and about half or two-thirds the 

 lateral diameter t of the crown. Lateral diameter of crown .23 inch, 

 bight of concave crown-face .15, hightof convex face .09 inch. 



We have seen but a single example of the present form, which pre- 

 sents the coronal region of the tooth nearly entire, but the fangs of the 

 base are broken away. Its relation to Dactylodus is inferred from the 

 general character of the crown and the apparently long, narrow base. 

 It seems to have closer relationship with the teeth described under the 

 name D. concavus of the St. Louis limestone, which is similarly excava- 

 ted in the concave face, but sufficiently distinct to be readily recognized, 

 the crown being relatively broader and not acuminate. From D. inflexus, 

 ~$. and W., of the same horizon, it may be distinguished by its narrow 

 base and fewer radicles. It may, however, prove to be a young or small 

 tooth of the latter form. Our knowledge of the variableness of these 

 forms is, as yet, insufficient to make it possible always to trace out the 

 specific identity of variable individuals. 



Position and locality : Upper fish-bed of the Chester formation ; Ches- 

 ter, Illinois. 



