364 PALEONTOLOGY OF ILLINOIS. 



Thus we might expect to find the L. curttis associated with G. pusillus, 

 and the L. selluliformis with Chomatodus insignis, Leidy. But as we 

 have no facts leading to the positive demonstration of such relationship, 

 and as they constitute amongst themselves a well-defined type, we have 

 preferred to recognize their apparent distinctness in preference to 

 assuming a probable generic and specific indentity with the above men- 

 tioned forms. 



LlSGODTJS CURTUS, St. J. and W. 



PL X A, Fig. 20-22. 



Teeth very small, lateral diameter and hight about equal, robust, 

 symmetrical in form. The crown forms a prominent, regularly arched, 

 or slightly acuminate, sharp ridge, presenting from above a semi-oval 

 or subelliptical outline; the convex face is sublenticular in outline, 

 vertical, nearly plane or slightly arched laterally, inferior angle gently 

 or broadly arched downward in the middle, sharply beveled to the 

 basal region, and bearing two comparatively strong coronal folds ; 

 concave face about one third higher than the opposite face, elliptical 

 in outline, crest and basal line nearly equally arched, gently concave 

 vertically, and more or less arched or convex laterally, basal margin 

 prominent, abruptly curved downward from the extremities, and gently 

 arched upward in the median region, and occupied by two or three well- 

 marked imbrications ; both coronal faces are enveloped in a dense, 

 polished enamel layer, except along the crest, which is usually more or 

 less worn and beveled along the concave side, in some instances it is 

 worn down to half its original elevation, presenting a long, narrow, 

 triturating area, roughened by the exposed extremities of the tubular 

 structure. The base varies somewhat in depth, the smaller the tooth 

 the greater is its proportionate depth; but in all cases it equals the 

 hight of the convex crown-face, and in the small teeth it is as deep 

 again, somewhat narrower and thinner than the crown, and apparently 

 always nearly vertical to the crown, broadly channeled in either face, 

 the one face being prominently defined from the projecting margin of 

 the concave crown-face, lower angles well-defined in perfect specimens, 

 inferior surface fiat and slightly beveled to the convex edge. A speci- 

 men of large size measures in lateral diameter of crown .36 inch, entire 

 hight of tooth .26, elevation of convex crown-face .15, depth of base in 

 the same side .11, hight of concave crown-face .18, lateral diameter of 

 base nearly .30 inch. A specimen of much smaller size is .18 inch in 

 lateral diameter of crown, which is equal to the entire hight of the 

 tooth, showing a relatively much deeper root than in the large teeth. 



This elegant little form is not uncommon in the fish- bed horizon of 

 the Upper Burlington, and is quite numerously represented in all the 



