308 PALAEONTOLOGY OP ILLINOIS. 



From 0. minutus, IS. and W., with which it is most intimately allied 

 of all its associates, it differs in the relatively thinner crown, also in the 

 less numerous and stronger vertical ridges; but in the character of the 

 basal plicse it strongly resembles the above species. In 0. minutus the 

 vertical ridges of the crown show a tendency to decussate, recalling a 

 similar style of coronal sculpturing observed in the form called O.ornatus, 

 N. and W.; but in the present teeth we fail to detect the slightest 

 semblance of similar decussations, although it is barely possible they 

 may have become obsolete from wear. Orodus parvulus of the St. Louis 

 limestone, perhaps, bears the most striking resemblance to the present 

 form ; it is, however, distinguished by the slighter and more numerous 

 vertical ridges, and deeper base. 



Position and locality : Bare in the upper fish-bed horizon of the 

 Keokuk limestone; Bentonsport, Iowa. 



Orodus neglecttjs, St. J. and W. 



PI. Vr, Pig. 26. 



Teeth of medium size, robust. Crown more or less strongly arched 

 vertically and moderately so backward, culminating in an obtuse, 

 swollen median prominence, at the base of which, in front, occurs a 

 slight callosity, lateral portions marked by obscure mammilloid undula- 

 tions with an indistinctly defined, angular crest, sharply constricted 

 along the basal borders, from which ascend with more or less regidarity 

 strong vertical ridges; in worn surfaces the crown presents a coarsely 

 punctate structure. Base oblique to the vertical plane of the crown, 

 posterior surface slightly convex and forming a rather broad area pro- 

 jecting bebind and parallel with the base of the crown, deeply excavated 

 in front, with broad beveled inferior surface ; both sides coarsely rough- 

 ened and pitted, inferior surface comparatively smooth. Greatest diam- 

 eter .50 to .75 inch. 



The present form is apparently closely allied to the teeth described 

 under the name 0. variocostatus from the Upper Burlington, also 0. gibbo- 

 sus (N. and W. sp.) of the Keokuk division ; it is, however, distinguish- 

 able from those forms by the greater regularity of coronal contour, and 

 the more regular disposition of the vertical ridges. 



Geological position and locality : It is a very rare form, only two speci- 

 mens being known to us, one from an arenaceous bed in the St. Louis 

 division of Monroe county, 111., and the other (represented in PI. VI, 

 Fig. 26) was obtained by Mr. Van Horne, in the calcareous shales of the 

 same formation at Pella, Iowa. 



