372 PALEONTOLOGY OF ILLINOIS. 



a large size specimen about .53 inch, anteroposterior diameter .20, or 

 but a trifle more than the bight of the concave crown-face, elevation 

 of convex face .05, depth of root beneath the concave face .14 inch. 



The individuals of the present form are somewhat variable in outline, 

 and in many of the teeth some of the characters noticed above are 

 obsolete; very few examples show any trace of coronal folds along the 

 basal margin of the convex crown-face, though all present, the same 

 rugose coronal structure and absence of coronal folds in the concave 

 face; the crest also varies from a vertical position in relation to the 

 horizontal plane of the crown, being sometimes slightly inclined from 

 within, which, however, is probably due to the wearing away of the 

 convex surface ; the crest rarely shows distinct denticulations, and in 

 some teeth the inferior region exhibits evidence of greater or less abra- 

 sion along its outer margin, as though by the impingement of the con- 

 tiguous tooth, to which may be attributed certain irregularities in the 

 basal contour. But in all these teeth there prevails a marked uniformity 

 in general appearance, requiring but slight experience to be able to 

 recognize their identity. 



This form is closely allied to that described from the Chester division 

 under the name T. depressus ; yet, notwithstanding in both may be 

 traced the same kind of individual variability, a critical comparison of 

 a fine suite of teeth from the two horizons discloses certain slight but 

 persistent differences, which together strongly impress one with the 

 conviction of their specific distinctness. In all the St. Louis teeth the 

 crest is less elevated and less erect or produced outward over the 

 basal line of the convex crown-face ; the superficial markings of the 

 crown also differ, in the present form being coarser and rougher than 

 in the Chester teeth. But in these teeth one cannot fail to observe 

 those appearances which seem to indicate the action of abrading influ- 

 ences to n, more considerable extent than is presented in the latter 

 form ; and in this feature, too, the same uniformity prevails as observed 

 in other respects. 



Begarding the affinities of these teeth with apparently allied forms 

 from the same horizons, the same suggestions are presented and the 

 same difficulties encountered in determining the nature of their resem- 

 blances, aud the character of their relationship, as has been observed 

 in connection with T. depressus. . However remote their resemblance 

 may be to T. sculptus, it is of the same nature as that of T. depressus to 

 T. polymorplius. In the present instance the resemblance is most strik- 

 ingly portrayed in the general contour of the basal region, the same as 

 in the Chester forms ; but the most thorough comparison of the mate- 

 rials in hand fails to prove their identity, while their relationship to 

 T. pumilus seems to be even more remote than in the case of T. depressus 



