VERTEBRATES, 43 



forms which we have described, such as A. cucullus, etc., are close, as is evinced 

 by their general resemblance of form and the perfect correspondence in all their 

 homologous parts ; but in A. politics the thickened edge of those species with 

 which we have compared it is truncated and even sulcated, the margins of the 

 sulcus being composed of denser tissue and wearing less rapidly. The office of 

 the tooth was plainly that of a grinder, the triturating surface being fitted for its 

 duty as in the ruminants, rodents, pachyderms, etc., by the alternation of harder 

 with softer bands of dental tissue. 



The entire tooth is excessively hard and dense, showing an interesting adap- 

 tation to the purposes it was intended to subserve. 



Figs. 2, 2 a represent the posterior aspect and profile of an average sized 

 tooth, natural size. 



Formation and locality: Keokuk limestone, Warsaw and Nauvoo, Illinois. 



Antliodus minutus, N. and W. 



PI. Ill, Figs. 3, 3 a, 3 6, 



Teeth very small and thin ; root rudimentary or obsolete; 

 crown elliptical in outline, cutting edge uniformly arched and 

 finely crenulated; posterior surface occupying half the height 

 of the tooth, concave vertically and horizontally, smooth and 

 polished throughout except along the superior margin, where 

 it is striated to the cutting edge; posterior basilar imbricated 

 folds 4-5, distinctly marked and occupying the lower half of 

 the posterior surface ; anterior face of crown very narrow, form- 

 ing scarcely more than one-fourth of the antero-inferior surface, 

 inclined to the inferior surface at an angle little greater than a 

 right angle, smooth and polished, terminating below in a single 

 prominent basal ridge, which is arched parallel with the cut- 

 ting edge ; inferior surface of the tooth roughened for its attach- 

 ment to the integuments, and showing, in a few low tubercles, 

 the rudiments of the root; length 6 lines; height 3 lines. 



This beautiful little tooth is remarkable for the entire absence of a root. It 

 seems to have been attached to the jaw by a roughened surface, occupying the 

 interval between the anterior and posterior coronal folds, which are very widely 

 separated. The character would exclude the species from the genus Petalodus, 



