232 IOWA GEOLOGICAL SURVEY 



Complete dentition in upper and lower jaws consisting appar- 

 ently of a single compound plate, each composed of one median 

 and two laterally disposed pieces, the sutures between them 

 deeply insunken in the form of a trifid sulcus in immature and 

 unworn examples, but becoming more or less obliterated in worn 

 and effete specimens. Functional surface as a whole moderately 

 convex, apart from the sulci referred to, and perfectly smooth 

 and polished throughout, without even vestigial traces of dentic- 

 ulate ridges. 



Great numbers of the fused dental plates belonging to this 

 species have been obtained from a comparatively thin cherty 

 layer at the old State Quarry near North Liberty, Iowa, where 

 they rival the detached tritors of Ptyctodus in abundance. Like 

 the latter, too, they exhibit a highly diversified aspect attribut- 

 able to the effects of wear during life, post-mortem abrasion, 

 chemical corrosion, and other familiar deforming agencies. In 

 young stages, sutures between the component parts are tolerably 

 distinct; but as the triturating surface becomes worn with use, 

 and is replaced by fresh secretion from below, the sulci become 

 shallower, fainter, and finally all but obliterated in effete ex- 

 amples. An endless series of individual variations is occasioned 

 by slight differences in the hardness of the several parts of the 

 functional surface, and the interaction of upper and lower denti- 

 tion accentuates these irregularities. In this way are to be ex- 

 plained the circular or elongate median concavities seen in worn 

 teeth where the longitudinal sulcus has been broadened. Very 

 often these shallow depressions are bounded along the sides, 

 sometimes also in front, by obtuse ridges ; and not infrequently 

 the forward part of the central excavations referred to is marked 

 by two well separated small circular pits (as seen in Plate IX, 

 Figs. 31, 34, 36, 38, etc.). 



Means are lacking for satisfactorily distinguishing between 

 upper and lower dental pavements, hence it is safe to affirm that 

 the compound plates of opposite jaws were constructed after es- 

 sentially the same pattern. A glance at the accompanying illus- 

 trations will satisfy one as to the extremely diversified appear- 

 ance presented by the general run of specimens found at the 

 typical locality. 



