380 PALEONTOLOGY OF ILLINOIS. 



dwell more at length on the features which the individuals of the pres- 

 ent form possess in common with those of other supposed congeneric 

 forms. 



Position and locality : From the lower fish-bed of the Chester 

 formation ; Chester, Illinois. 



Tanaodus polymorphus, St. J. and W. 



PI. XX, Fig. 17-19, 24 



Teeth small, variable in outline from irregularly-circular to broad- 

 elliptical, the transverse sometimes exceeding the lateral diameter of 

 the tooth. Crown but moderately elevated and arched along the crest, 

 which is obtuse and faintly denticulated, one or other, sometimes both, 

 of the lateral extremities broadly rounded or obtusely angular; convex 

 . face low, generally projecting beyond the basal line, though sometimes 

 inclined from within, often slightly channeled, more or less strongly 

 arched laterally according to the outline of the crown, basal margin 

 nearly horizontal or gently arched downward in the middle, and 

 again very slightly curved downward on n earing the lateral angles, 

 and marked by a narrow band of two to four or five quite regular 

 imbricating folds; the concave face is much more variable in outline 

 and contour, subcircular or elliptical, very gently ascending to the crest, 

 nearly plane or hut gently depressed, in some instances presenting a 

 slight convexity in the lower median region, basal margin closely agree- 

 ing with the curvature of the crest, though perhaps never as strongly 

 arched, in the more oval individuals forming an obtuseangleinthe median 

 region, and bordered by a usually well-marked coronal baud composed 

 of from three to five regular, vertically striated imbrications, which are 

 continuous with those of the opposite face though relatively wider. 

 In well preserved specimens the coronal surfaces present a delicate 

 veruiiculose ornamentation in the convex face, also in the median por- 

 tion of the concave face, the lower portions of which are beautifully 

 marked with more or less numerous, regular rugae which arise in the 

 upper margin of the coronal belt, deflected laterally or converging in 

 their upward course, becoming obsolete in the middle region, and which 

 are quite distinct from the vertically elongated punctae which often 

 appear in the worn surfaces along the crest. The root forms a narrow 

 sharp-edged, wedge-shaped process, in nearly the same vertical planes 

 as the crown and which is reduced almost to a reversed conical condi- 

 tion in short, oval teeth, inbeveled and slightly channeled laterally 

 along the abrupt face beneath the basal margin of the concave crown 

 face, with which it forms an acute or right angle, while its opposite face 



