VERTEBRATES. 381 



forms a nearly uninterrupted plane with the basal area, which latter is 

 slightly bulging iu the middle, smooth, and meeting at an obtuse angle 

 •with the interior border of the eouvex crown-face; the proportionate 

 depth of the root varies considerably, nor does this variation seem to be 

 dependent upon any particular form or size of tooth. A specimen of 

 large size measures in lateral diameter .42 inch, anteroposterior diame- 

 ter .33, or nearly that of the concave crown face, hight of convex face 

 .09, depth of abrupt face of root about .11 ; while an exceedingly min- 

 ute oval tooth has the transverse and lateral diameters of the crown 

 nearly equal, or about .OS inch, the aprupt face of the root slightly 

 exceeding .OS inch in depth, showing a remarkably produced wedge- 

 shaped process. 



The collection of Mr. Van Horne contains a beautiful suite of the 

 above designated form, many of the individuals of which are so per- 

 fectly preserved as to afford a very satisfactory comparison with other 

 and allied forms. It is very closely related to the teeth which we have 

 described from the St. Louis formation under the name T. sculptus, pre- 

 senting even a greater individual diversity than has been observed iu 

 the latter form. In order to appreciate the differences, however slight, 

 which distinguish the individuals of these forms, it is not necessary to 

 select some particular example, nor does the possession of a large vari- 

 ety of teeth render one's judgment and conclusions confused and 

 unsatisfactory; on the contrary, in connection with the data which has 

 been carefully noted bearing on the association of the teeth in the two 

 horizons, the distinctive peculiarities pertaining to each, though slight, 

 appear to be persistent and readily recognizable. Compared with T. 

 sculptus, with which form alone it is likely to be confounded, in the 

 present form the convex face is less uniformly depressed, the coronal 

 folds generally less prominent, and the rugae and vertical striae coarser ; 

 also the root is stronger, generally deeper, and more nearly vertical or 

 less obliquely deflected outwards. The style of coronal ornamentation 

 is essentially the same in all varieties of the present form, and the 

 gradation of the individuals is of a nature that furnishes the best, evi- 

 dence accessible of the specific identity. 



As to their affinities with the preceding forms, T. depressus and T. 

 bellicinctus, of the same horizon, the present teeth hold precisely the 

 same relationship as observed in the case of T. sculptus with T. prce- 

 nuntins and I. pumilus of the St. Louis formation. But iu the present 

 state of our knowledge of these forms, it would seem mere presumption 

 to recognize their specific identity, though such may prove to be the 

 actual fact. 



Position and locality: Lower fish-bed of the Chester formation; 

 Chester, Illinois. 



