290 PALEONTOLOGY OF ILLINOIS. 



excavated ; coronal cusps nearly equal in size, terminating in a sharp 

 point, -with sharp lateral edges, lenticular in transverse section, more 

 compressed in front than behind, the exterior pair diverging and 

 recurved, central cone erect or laterally deflected, and ornamented with. 

 a very few sharp, irregularly disposed costre in the outer faces. Hight 

 of medium size tooth .05 inch, and .07 inch between the tips of the 

 exterior denticles. 



The few individuals showing the crown entire present the appearances 

 described above. But the collections afford several imperfect teeth, 

 apparently referable to the same species, which show the base consid- 

 erably produced posteriorly, flattened and slightly twisted, precisely in 

 the same manner as in the allied forms occurring in the succeeding 

 Burlington and Keokuk divisions. In case of this identity, the laterally 

 elliptical outline of the base, as described above, is probably due to 

 mechanical abrasion. 



These teeth are most intimately allied to the form which occurs in 

 the Burlington limestone, Thrinacodus incurvus, (N. and W. sp.) from 

 which, indeed, they are chiefly distinguishable by their exceedingly 

 minute size. The costse seldom exceed three in the outer face of the 

 coronal cusps, the inner face showing obscure traces of fine and more 

 numerous striae. 



Geol. position and locality : Not uucommon in the upper fish-bed of 

 the Kiuderhook division ; Burlington, Iowa. 



Genus MESODMODUS, St. J. and W. 



Teeth laterally elongated. Base consisting of one inferiorly flattened, 

 posteriorly obliquely produced, massive plate, of which the posterior 

 face slopes downward and slightly backward at an obtuse angle to the 

 posterior crown face, anterior face slightly produced along the shoulder 

 which extends parallel with the base of the crown, vertical or beveled, 

 and occupied by a more or less prominent median protuberance which 

 extends to the edge of the inferior surface; both faces more or less 

 roughened or pitted, lateral angles truncated or rounded and more or 

 less constricted above, equaling the lateral diameter of the crown. 

 Crown rising along the anterior border, sharply constricted in front and 

 laterally, and well defined, sometimes constricted, from the posterior 

 basal face, nearly equaling the base in anteroposterior diameter, but 

 more or less compressed along the crest, which rises into a more or less 

 prominent median or submedian cusp, vertical or laterally deflected and 

 recurved, usually compressed, with distinct, sometimes sharp cutting- 

 edges; the lateral portions of the crown are more or less strongly den- 

 ticulated, the extremities bearing slightly more prominent cusps than 



