378 PALEONTOLOGY OF ILLINOIS. 



marked and apparently persistent. The root, in specimens which have 

 been subjected to extreme abraiding action, often shows a tendency to 

 divide into radicles which, however, differ from those characteristic of 

 PolyrMzodvs in their extremely irregular disposition, plainly indicating 

 their accidental origin — the result of the attenuation of the originally 

 thick root and the deepening or exposing of the irregularly spaced 

 vertical sulci which appear in nearly all Petalodont forms. 



Geological position and locality : From the lower fish bed in the 

 Chester division ; Chester, Illinois. 



Tanaodtts depkessus, St. J. and W. 



PI. XI, Fig. 11-13. 



Teeth small, of strong proportions, curvilinear-elliptical as seen from 

 above, though variable in outline, the longer sides nearly parallel or 

 sometimes gradually converging towards one or other extremity, which 

 latter are oblique, rounded or obtusely pointed. Crown depressed, con- 

 cave face faintly excavated vertically and more or less considerably 

 arched laterally, rarely plane, inferior margin direct, or usually gently 

 arched inward along the greater median extent and obliquely truncated 

 or rapidly rounded to the lateral angles, gently or broadly arched along 

 the crest, which generally projects beyond the inferior line of the convex 

 face, forming a more or less well-defined obtuse angle along the middle 

 portion, but gradually diminishing in elevation towards the extremities 

 where it is sometimes reduced to a simple convexity but slightly raised 

 above the general plane of the crown, often obscurely denticulated in 

 the prominent part; convex face very low, laterally arched, slightly 

 channeled, inferior margin irregularly and very gently curved upward 

 in the median region, gently or acutely rounded at the extremities, in 

 perfect teeth sharply defined from the basal region ; in a few examples 

 the convex coronal margin presents traces of exceedingly delicate 

 imbricating folds, of which there are three or four, but in the opposite 

 basal margin the coronal folds are almost always obsolete — the fine ver- 

 miculose markings occupying the entire surface, except a narrow space 

 immediately along the crest which is coarsely striato-punctate. The 

 outline and contour of the basal area varies considerably and in the 

 same manner as does that of the crown, irregularly convex or concave, 

 forming a plane nearly at right angles to that of the convex crown- 

 face, and nearly continuous or uniform with the root, which latter, how- 

 ever, makes an abrupt descent from the concave coronal margin, faintly 

 channeled, laterally converging below, forming a strong wedge-shaped 

 process terminating in an acutely rounded edge, in depth equal to or 



