VERTEBRATES. 371 



ces of attrition, possibly of immaturity, which characterize the worn 

 teeth of the former species : indeed so intimate is the resemblance 

 between the worn examples of these two species, that, were they known 

 only from the imperfect teeth, they might be mistaken for one and the 

 same species. 



Associated with the present form there is a group of teeth equally 

 well represented, and which holds precisely the same relationships to 

 that above described, as do the teeth noticed under the head of T. 

 depressus to T. bellicinctus ; and which, for the same reasons as assigned 

 in that instance, are provisionally referred to a distinct group, which 

 is noticed at length in the description under T.prwnuntius. 



Position and locality : Upper beds of the St. Louis limestone ; Alton, 

 Illinois, Pella, Iowa. 



Tanaodus PrvEnuntius, St. J. and W. 



PI. XI, Fig. 6, 7. 8, 9, 10. 



Teeth small, laterally elongated, long-elliptical or gently arcnate, 

 outer and inner margins more or less nearly parallel, extremities 

 rounded or obliquely truncated. The concave crown-face is gently 

 depressed vertically, very slightly so or nearly straight in the long 

 diameter, and sometimes gently arched, basal margin slightly raised 

 and forming a right-angle with the abrupt face of the root, destitute 

 of distinct imbricating folds, straight or slightly arched inward and 

 upward, and more or less abruptly curved round the lateral extremities, 

 gently arched in both directions along the crest, which forms a low, 

 obtuse ridge, sometimes obscurely and irregularly denticulate, and 

 closely bordering the convex side of the crown, which latter is corres- 

 pondingly low, nearly plane or faintly channeled vertically, broadly 

 arched laterally, very gently and sometimes irregularly curved down- 

 ward along the lower margin, which is sharply defined from the inferior 

 region, and sometimes, though rarely, marked by obscure, irregularly 

 disposed, thread-like lines; the coronal surfaces present a coarse, 

 irregular vermiculose appearance, and along the crest a coarse punc- 

 tate or striato-punctate structure, the vertical tubes being often gathered 

 into irregularly spaced bundles, which give rise to the obtuse denticu- 

 lations frequently observed in the crest. The root forms a strong, 

 laterally narrowed, wedge-shaped process, faintly channeled laterally 

 and abruptly defined from the margin of the concave crown-face, upon 

 the opposite side scarcely definable from the wide basal area, the contour 

 of which is more or less irregularly convex, and defined above by a 

 faint sulcation extending along and just beneath the basal angle of the 

 convex crown-face; the basal area is faintly roughened or smooth, and 

 faint vertical sulci sometimes appear in the root. Greatest diameter of 



