294 PALAEONTOLOGY OF ILLINOIS. 



by one to three relatively prominent secondary cusps, which are con- 

 nected with each other and the median prominence by the sharp coronal 

 crest, very variable in occurrence, sometimes a pair on either extremity, 

 of which the exterior are smallest, sometimes one strong cusp on the 

 short extremity, and three in the long opposite wing, agaiu, the one 

 extremity being formed by the even prolongation of the sharp lateral 

 edge of the median cusp, with only a single denticle upon the opposite 

 extremity ; surfaces enveloped in a polished enamel like coating, and 

 marked, especially in the basal half, by a few short, sharp, thread-like 

 vertical lines, which become obsolete in the upper portions of the crown. 

 Lateral diameter of a symmetrical medium sized specimen . 19 inch, 

 antero-posterior diameter about . 07, greatest elevation of tooth . 09 

 inch; some specimens are proportionately shorter and more considera- 

 bly produced posteriorly, others are narrower, but the specimen mea- 

 sured probably represents the normal proportions of the teeth. 



We have examined but few examples of the form described above, 

 which, however variable among themselves, exhibit a persistence in 

 coronal markings which readily confirms their specific identity. They 

 bear, at least in some examples, even closer resemblance to M. ornatus 

 of the Upper Burlington, than they do to M. exsculptus, with which lat- 

 ter they are associated. It seems barely possible, indeed, that they may 

 pertain to the latter form ; but their distinctive features, as exhibited 

 by the comparatively smooth coronal surfaces and sparsely denticulate 

 crest, and the broader basal support, do not harmonize with the varia- 

 ble characters of the latter form, while their individual variation is quite 

 as marked, though apparently never approaching that of ill", exsculptus 

 sufficiently to furnish satisfactory evidence of their specific identity. 



Position and locality : Eare, in the lower fish-bed of the Kinderhook ; 

 Burlington, Iowa. 



Mesodmodus ornatus, St. J. and W. 



PI. V, Fig. 12-14. 



Teeth very small, lateral diameter about equal to twice the hight. 

 Base subcrescentiform in outline, broadly produced posteriorly and 

 rouuded, gently concave in front, anterior face low, beveled, roughened, 

 protuberance median or submediau, moderately prominent, posterior 

 face gently convex laterally, forming an obtuse angle with the posterior 

 crown face, finely punctate, inferior surface slightly oblique to the hori- 

 zontal plane of the crown, relatively broad, plane or faintly concave, 

 gently arched upward, lateral extremities sharply rounded in front. 

 Crown relatively strong, prominent, more or less strongly produced in 

 front and constricted iuferiorly and laterally, well-defined from the base 



