282 PALEONTOLOGY OF ILLINOIS. 



lateral angles sharp, though generally obsolete from wear, outer face 

 marked by rather fine, irregularly disposed sharp-crested cariuae, inner 

 face apparently smooth, the enamel-like layer terminating inferiorly 

 in a faintly defined border. A specimen of ordinary size measures in 

 greatest diameter of base .45 inch, lateral diameter .24, depth .15, hight 

 of cone about .30, greatest diameter at base .14, lateral diameter .11 

 inch. 



The teeth of this form present considerable individual variation, 

 which appears to be mainly attributable to wear. We have a few speci- 

 mens from the Keokuk limestone, which are very intimately allied to 

 the form especially referred to above, to which we have provisionally 

 referred them, as a variety, under the designation L. robustus. The 

 latter teeth are uniformly more robust, and, although the base has the 

 same outline viewed from above, it is deeper and sometimes strongly 

 angular or heeled in the inferior surface, instead of being plane, as 

 almost always occurs in the Upper Burlington teeth ; the anterior 

 extremity of the base is also more prominently produced, while the form 

 of the cone, which in the few examples examined is worn smooth, is in 

 no marked degree unlike that of the form described. The differences 

 between these forms are strikingly similar, in degree, to those which 

 distinguish the teeth of Cladodus micropus, N. and W., in the same hori- 

 zons ; and it is not, perhaps, unreasonable to look for a greater intimacy 

 in the relationship of these two groups, than the data possessed at this 

 moment would seem to justify. 



Position and locality. — Not uncommon in the fish-bed of the Upper 

 Burlington, Louisa and DesMoines counties, Iowa, and localities in the 

 same horizon in Illinois. The Keokuk form occurs at Warsaw, Illinois, 

 Keokuk and Bentonsport, Iowa, and Lagrange, Missouri. 



Lambdodtjs transversus, St. J. and W. 



PI V, Fig. 4. 



A single specimen of a minute tooth from the St. Louis limestone, 

 apparently referable to the above genus, is distinguished by its rela- 

 tively great lateral diameter of base, which is two and a half times 

 greater than its antero-posterior diameter, comparatively thick, laterally 

 oval in outline, anterior border nearly equally produced with the post- 

 erior margin, and beveled to a thin edge, coarsely roughened. The 

 coronal cusp is very strong, rapidly tapered, moderately recurved, 

 antero posteriorly compressed, lenticular in transverse section, lateral 

 angles acutely rounded, surface smooth. Hight of tooth .07 inch, cone 

 nearly central, lateral diameter of base .15, antero-posterior diameter 

 .06 inch. 



