360 PALAEONTOLOGY OF ILLINOIS. 



to the crown, a little narrower and not as thick, in depth usually less 

 than, though in some instances nearly or quite equal to, the hight of 

 the crown, both faces slightly channeled laterally, lower edges worn or 

 less angular, inferior surface gently beveled to the concave side. A 

 very acuminate specimen is .20 inch in greatest lateral diameter, hight 

 of crown .14 ; a low, serrated tooth measures in lateral diameter .18, 

 entire hight of tooth .17, elevation of crown .OS. There are individuals 

 with even a lower crown, but that last mentioned probably represents 

 the normal proportions of these teeth. 



This form is much like that described under the name L. curtus, 

 presenting nearly the same general outline and proportions. It differs, 

 however, in the serrated character of the crest, and the medially upward 

 arching or horizontal basal line of the convex face, also in its smaller 

 size. Otherwise it is most intimately allied to the above mentioned 

 form, in the serrated condition of the crest does it depart most widely. 

 Yet we find examples in which the latter character is nearly obsolete, 

 and which, but for the slight lateral position of the apex, as well as 

 those other distinctive features noted above, might be mistaken for the 

 form with which it occurs. From L. selluliformis of the St. Louis lime- 

 stone, it differs in its less symmetrical outline and proportions, and the 

 vertical position of the base ; in the character of the coronal serration 

 it closely resembles that form. 



"We are largely indebted to Mr. Springer for the means of compar- 

 ing a large number of individuals of this and the previously described 

 form, all of which were obtained from a single locality in the ichthyic 

 stratum of the Upper Burlington limestone ; the same association is 

 also known at other localities, but the Louisa county (Iowa) locality has 

 afforded by far the largest and most interesting suite of these teeth. 



Position and locality : Fish-bed of the Upper Burlington limestone; 

 Louisa and Des Moines counties, Iowa; Warren county, Illinois. 



Lisgodus selluliformis, St. J. and W. 



PI. Xa, Fig. 16. 



Teeth very small, thick, greatest hight exceeding the lateral diameter. 

 The crown is strongly arched, sharp, terminating in a thickened, more 

 or less acutely rounded point, and low down upon the flanks one or two 

 faint serrations give to the lateral edges an obscure denticulate charac- 

 ter ; the convex crown face presents a subtriangular, vertical area, 

 sometimes moderately arched laterally, and margined below by a well- 

 marked horizontal coronal fold ; concave face obtusely curvilinear in 

 outline, broadly rounded below, gently concave vertically, its greater 

 area occupied by a strong median ridge descending from the apex and 



