344 PALAEONTOLOGY OF ILLINOIS. 



like lines, wbich increase by implantation and are much disconnected 

 on approaching the crest, and much more strongly developed in the 

 concave face where tbey reach half way to the upper edge of the coronal 

 belt, enveloped in a polished enamel like layer. Eoot apparently stout, 

 tbick, somewhat rapidly converging toward the inferior border, at its 

 origin about half the lateral diameter of the crown. Lateral diameter 

 of crown .16 inch, bight of concave face above the coronal belt .10 inch, 

 including the coronal belt .14 inch, antero-posterior diameter .05 inch. 



The form under consideration is known to us by only a single speci- 

 men, an exceedingly handsome little tooth, which is nearly perfect with 

 the exception of the root, wbich latter, unfortunately, bas been broken 

 away. In its general aspect it recalls the form referred to under the 

 name Desmiodus ligoriiformis, from the same geological position, the 

 resemblance being especially striking in the character of the coronal 

 belt encircling the broad, inbeveled base of the crown, together with 

 the superficial ornamentation. It differs, however, markedly in the outer 

 coronal aspect, as its less angular outline, the regular arching of the 

 crest with its peculiar irregular serration, and the absence of the strong 

 median ridge in the convex face ; in each and all of which particulars 

 it presents a striking contrast to the above mentioned form. The root 

 was probably very short and stout, and set in nearly the same vertical 

 plane as the crown. The variations of the individuals of D. ligoniformis 

 present no approximation to the peculiarities apparent in the present 

 form ; while our material is insufficient to demonstrate, perhaps, their 

 generic identity, much less their specific identity, by supposing these 

 two forms to represent the dental .armature of opposite jaws of the 

 same species. 



Position and locality : Discovered by Dr. Williams in the upper 

 fish-bed of the Keokuk limestone ; Boonville, Missouri. 



Genus YEtfUSTODUS, St. J. and W. 



Teeth laterally elongated, vertically arched. Crown abruptly con- 

 stricted at the base and defined by imbricating folds, wbich form a pro- 

 portionately broader belt along the concave side than in the opposite 

 border, crest moderately elevated and usually well-defined, uniform or 

 rising into a strong, obtusely conical, more or less eccentric median 

 prominence, lateral portions more or less distinctly marked by obtuse 

 denticulations, which gradually diminish in size towards the extremi- 

 ties ; coronal surfaces enveloped in a polished enamel-like layer. Base 

 forms a broad, shallow plate, more or less obliquely flattened inferiorly 

 with well defined angles in front and behind, lateral and antero-post- 



