9-4 PALAEONTOLOGY OF ILLINOIS. 



Aspidodus convolutus, N. and W. 



PI. VIII, Figs. 12, 12a. 



Teeth robust, ovoid in outline as seen from above, thick and 

 massive, strongly revolute, having a cylindroid figure ; upper 

 surface without, revolving ridges or furrows, finely striated 

 transversely, uniformly filled throughout; under surface nearly 

 smooth. 



This singular tooth scarcely corresponds to any other in the collection or 

 heretofore described. In the generalities of its form and structure it approaches 

 most nearly to the revolute forms of A. crenulatus, and as it is derived from the 

 same locality, we have thought best to place it provisionally in the same genus 

 with that species. The only specimen contained in the collection is considera- 

 bly broken, so that its entire figure can not be determined from it. It is, 

 however, so conspicuous, from its size and form, that it seems desirable that it 

 should be published in order that it may be of use for geological purposes. 



Figures 12, 12a, respectively, represent the upper surface and profile section, 

 natural size. 



Formation and locality: Chester limestone, Chester, Illinois. 



Pcecilodus rugosus, N. and W. 



PI. VIII, Fig. 13. 



Tooth oblong or sub-elliptical in outline, posterior extremity 

 sub-acute, anterior truncated, strongly revolute, with two promi- 

 nent revolving ridges, separated by a deep central furrow ; 

 crown surface highly ornamented with numerous sub-equal 

 folds of enamel, running nearly parallel with the basal margin. 

 Where unworn these are angular carinae, with one broad and 

 one narrow, almost perpendicular, sides; upon worn surfaces 

 they form rounded, transverse ridges, which undulate its out- 

 line. The enamel pores are uniformly distributed over the 

 entire surface. Length, 1 inch, 6 lines; breadth, 8 lines; enam- 

 eled folds, half a line in breadth. 



