VERTEBRATES. 353 



the belt, where they are upturned at their extremities, and becoming 

 obsolete before reachiug the lateral extremities of the crown, where the 

 main belt is more or less suddenly deflected, sometimes at right-angles, 

 and curved round the extremities. 



In all these varieties the base forms a thick, more or less angular 

 plate, the inferior surface of which is slightly oblique to the horizontal 

 plane of the crown, from which it is strongly defined on either side by 

 a deep sulcation, and constituting less than half the vertical elevation 

 of the tooth. The crown is highly polished, but in worn specimens the 

 crest is more or less distinctly punctate, in which condition the lateral 

 denticulations are often nearly obsolete. 



The preseut form was first recognized in a few specimens in the col- 

 lection of Prof. Worthen, all of which represented the small arched 

 teeth. Subsequent discoveries brought to light larger and more sym- 

 metrical teeth, corresponding to the varieties in the St. Louis limestone 

 which have been specifically associated under the name V. Leidyi. 

 Some examples among these teeth are remarkable for the prehensile 

 character of the strong apiculate median cone, and this feature is con- 

 fined apparently to certain of the more strongly arched specimens, and 

 has led to their reference to the anterior portion of the jaws, recalling 

 the acuminate teeth occupying a similar position upon the jaws of Ces- 

 tracion. Others, again, though strongly arched, have the median cone 

 nearly vertical and angularly buttressed in the convex face, the lateral 

 denticulations being less prominent, and the concave face more nearly 

 approximating its condition in the last above described variety. 

 Although, regarded separately, these varieties may appear to be char- 

 acterized by persistent differences, yet, when we bring to our aid the 

 evidence afforded by other forms of the type, one can hardly fail to 

 recognize in these seeming differences the local and individual varia- 

 tions of the teeth of one and the same species. The evidence which 

 may hereafter be obtained from a more complete series of specimens, 

 will as likely prove the specific identity of these varieties as is the result 

 in the numerously represented and equally and similarly variable denti- 

 tion of the previously described forms. 



The present form is readily recognized from the species occurring in 

 the subjacent St. Louis formation, by r the more regular disposition of 

 the imbricating folds ; indeed, it bears a more intimate resemblance to 

 the forms described from the Keokuk and upper Burlington formations 

 than to V. Leidyi. 



Position and locality : Not uncommon in the upper fish-bed of the 

 Chester limestone; Chester, Illinois. 



