318 PALEONTOLOGY OF ILLINOIS. 



the median cone being very obscurely developed, lateral portions of the 

 crest obscurely and irregularly denticulate ; the concave face is also 

 sometimes more or less distinctly undulated in a manner similar to that 

 observed in the large teeth, with which they otherwise bear features in 

 common. Of the latter style of teeth the collections contain few exam- 

 ples, and this is true in relation to all the other forms of the genus, 

 with the exception of V. Leidyi, of which the collection of Mr. Van 

 Hoexe affords a fine series of both the large median teeth and the 

 anterior teeth, and the variety represented by the teeth just described. 

 In regard to the geographical distribution of the present form, it is 

 known to occur at nearly all the localities in the Upper Burlington 

 where the fish-bed stratum is exposed. A noticeable feature of these 

 collections is the persistency of the characters which distinguish, even 

 though it appear slight, the Upper Burlington from the Keokuk teeth, 

 and the present form from that previously described, with which it is 

 associated at the locality which has received the most exhaustive explo- 

 ration. But, however similar species may appear when comparisons are 

 limited to a few examples of one or other form, experience oftener than 

 otherwise proves, on the examination of large materials, a much greater 

 divergence in the characters peculiar to each than it was possible to 

 detect in a comparatively limited series of specimens. 



Position and locality : Common in the Upper Burlington fish-bed ; 

 Louisa and DesMoines counties, Iowa, and Henderson, Warren and 

 Adams counties, Illinois. 



Yenustodtts tentjtceistattjs, St. J. and W. 



PL IX, Fig. 19-24. 



Teeth attaining medium size, variable in outline and proportions, in 

 which latter respect at least three varieties may be recognized. 



The first of these attains the largest size and most robust proportions, 

 ranging from .35 inch to 1 inch in lateral diameter, and .10 to .33 inch 

 in antero posterior diameter, presenting from above an irregular long- 

 elliptical outline, sometimes sigmoidally curved, and moderately arched 

 between the extremities. Crown very gradually converging from the 

 median region towards the lateral extremities, which are obtusely 

 rounded, one narrower than the other, lateral portions of the crest low, 

 compressed, forming a sharp, more or less distinctly and numerously 

 denticulated crest, which is suddenly produced into a relatively strong 

 eccentric prominence, which in mature individuals is laterally com- 

 pressed, erect or laterally deflected, generally towards the short extrem- 

 ity, and usually flanked in the convex face by an angular vertical 

 buttress and a more or less prominent gibbosity in the opposite face, 



