112 PALAEONTOLOGY OF ILLINOIS. 



slight evidence, the existence of Chiniceroid sharks at the period of the depo- 

 sition of the Carboniferous limestone. 



Trigonodus major, N. and W. 



PI. XI, Pigs. 8, 9, 9 a. 



Teeth sub-triangular, with one longer and two shorter sides, 

 the latter including the best denned angle, near which is the 

 most prominent portion of the tooth, the shortest sides are sub- 

 equal somewhat curved, longer side more or less waved, and 

 irregular. The central prominence forms a broad arched sur- 

 face from which two obtuse ridges descend, the longer one to 

 one of the angles, the shorter one to the long and irregular 

 margin. From the central prominence the surface falls off 

 toward the opposite angle in a broad, shallow furrow; enam- 

 eled surface highly polished and finely punctate throughout; 

 length, 1 inch, 9 lines; greatest breadth, 1 inch; height, 7 

 lines; thickness at centre, 4h lines. 



Figures 8 and 9, represent a pair of teeth apparently from the opposite sides 

 of the same jaw; figure 9 a, the profile from the angled side, natural size. 

 Formation and locality: Burlington limestone, Quincy, Illinois. 



Trigonodus minor, N. and W. 



PI. XI, Figs. 7, la. 



Teeth half the size of T. major, sub-triangular in outline, 

 shorter sides curved, longer nearly straight. Of the small 

 angles one is rounded, the other sub-acute; the most prominent 

 point of the crown, half way between the obtuse and the 

 rounded angle, being the summit of the arched ridge which 

 connects them. Towards the acute lateral angle the surface 

 slopes down uniformly with no distinct sulcus. The surface 

 rising from the margin which connects the medial with the 

 rounded lateral angle marked with 4 or 5 sub-equal and par- 



