388 PALEONTOLOGY OF ILLINOIS. 



more abruptly curved upward to the opposite angle, and bordered by a 

 moderately wide and, in the present state of preservation, obscurely 

 imbricated coronal belt; concave crown-face somewhat deeply excavated 

 laterally and vertically, broadly and irregularly elliptical in outline, 

 basal margin irregularly broadly arched and produced beyond the basal 

 surface, sharply inbeveled, basal band wide, consisting of as many as 

 six imbricating folds, gently rounded at one extremity, more abruptly 

 so at the other, invested with a delicate, polished enamel layer, through 

 ■which is visible the miuute punctate structure; the opposite crown-face 

 is denuded of its enamel coating, presenting a fine granular structure, 

 and iu the crest striato-punctation. Base following the strong curva- 

 ture of the basal margin of the concave crown-face, considerably less 

 in lateral diameter, moderately thick, and obliquely produced in the 

 plane of the concave face, well-defined from the broad basal area, inte- 

 riorly rounded, and irregularly divided into six or eight radicles of 

 inequal size and shape. Lateral diameter of crown 1.37 inch, bight of 

 tooth .77, elevation of convex crown-face .42, lateral diameter of base 

 1.05 inch, depth beneath the basal border of concave crown-face .26. 



The above form was originally described from specimens in the col- 

 lection of Mr. Van Horne, but subsequently it has been found at two 

 or more other localities in the same horizon, and with the evidence 

 adduced by this additional material, the variations presented by indi- 

 viduals of the form do not appear to be remarkable. In some respects 

 it bears somewhat intimate resemblance to P. radicans, McCoy, with 

 specimens of which, from the Carboniferous limestone of Armagh, iu 

 the Museum of Comparative Zoology, we have had opportunity to make 

 direct comparison. Especially noticeable is this resemblance in the 

 broadly arched, produced basal margin of the concave crown-face; it 

 differs, however, in the more compressed crown, less robust and shorter 

 radicles. The basal area is quite wide aud plane, forming an acute 

 angle with the convex crown-face, instead of an obtuse, angle as in the 

 European form. From P. dentatus, 1ST. and W., of the Chester forma- 

 tion, it is distinguished by its broad elliptical outline, more compressed 

 and higher crown, in which character it presents even greater contrast 

 with P. Littoni, ~N. and W., with which it is associated. The latter form 

 is remarkable for its long-elliptical outline, which, however, may be 

 attributable, in greater or less degree, to the wearing down of the 

 convex aspect of the tooth by trituration ; yet the basal border of the 

 concave crown-face, which forms a nearly straight belt gently curved 

 outward at the extremities and sharply rounded, contrasts so widely 

 with the teeth above described that we can hardly believe these distinc- 

 tions represent merely individual variation- We have introduced illus- 

 trations of a remarkably fine example of the latter form, belonging to 



