48 PALAEONTOLOGY OF ILLINOIS. 



Dactylodus inflexus, N. and W. 



PI. Ill, Figs. 8, 8a. 



Teeth of small size, slightly broader than high; crown 

 equaling the root in height; cutting edge sharp, regularly 

 arched, unevenly striated and crenulated by the somewhat 

 irregularly disposed marginal enamel-tubes; posterior face 

 elliptical, strongly concave vertically, nearly straight laterally; 

 anterior face throughout all its central portion plain and 

 smooth, long-rhomboidal in outline, the upper and lower angles 

 rounded symmetrically, the lateral ones long-pointed, the 

 points being abruptly curved backward and downward to join 

 the coronal ridges. This face of the crown is much elevated, 

 forming an acute angle with the base; the anterior ridge divi- 

 ding the crown from the base is prominent, but like the spaces 

 bordering it very smooth. When worn, the anterior face of 

 the crown shows near the upper border a somewhat reticulated 

 porosity produced by the cutting across of the oblique enamel 

 tubes. The root is flat, somewhat constricted at its junction 

 with the crown, but expanded below and divided into 5—6 

 divergent and conspicuous radicles. 



Though sufficiently distinct from the two preceding, this species is closely 

 allied to them, forming the fourth with which we are now acquainted, of this 

 interesting group. Its most distinctive character is that which suggested the 

 specific name given it, in the strong backward inflexion of the upper part of 

 the crown. This gives it the appearance of having a flat summit, and of being 

 a grinding tooth. It has, however, a sharp, cutting edge, and its position on 

 the jaw must have been such that the summit of the crown was vertical or 

 slightly inclined toward the throat. The anterior face of the crown is worn 

 near the edge as though a corresponding tooth in the other jaw had matched 

 with this, and the two had worked together like the blades of a pair of shears. 



The form and length of the root in many Petalodonts is such as to indicate 

 that they could have had little motion, but were probably fixed in their places, 

 and like these worked together in the manner of scissor blades. 



Figure 8 and 8 a, represent the anterior face and profile, natural size. 



Formation and locality: Chester limestone, Chester, Illinois. 



