398 PALEONTOLOGY OF ILLINOIS. 



great size, more minutely denticulated crest, and regularly arched basal 

 margin, are sufficient to establish its distinctness from that form. 

 Position and locality : Keokuk limestone, Bentonsport, Iowa. 



Otenopetalus (Petalodxjs) bellultjs, St, J. and W. 



PI. XII, Fig. 9 



Teeth very small, oval or subelliptical in outline, rather strongly pro- 

 portioned. Crown moderately thick at base, sharp and regularly arched 

 along the crest, with acute lateral extremities; concave face elliptical 

 in outline, gently excavated, basal margin regularly arched downward 

 in the middle, and bordered by three more or less imbricated folds, 

 which expand in the median region to twice their breadth nearer the 

 extremities, the upper fold spanning the crown iu an irregular though 

 nearly horizontal direction, and but slightly produced below beyond the 

 plane of the basal region; convex face of irregular outline, long-ellipti- 

 cal or sublenticular, gently arched laterally, nearly plaue vertically, 

 basal margin irregularly curved downward, forming an obtuse angle, 

 and limited by two narrow imbrications, which are sometimes slightly 

 deflected at the extremities, but usually describe a slight sigmoidal 

 curvature. Base short, broadly rounded below, and obtuse, somewhat 

 distended in the middle, well-defined at the lateral angles, in breadth 

 considerably less than that of the crown, with which it is nearly in the 

 same vertical plane. Greatest lateral diameter of crown of large-sized 

 tooth .25 inch, hight of entire tooth .15, elevation of concave crown-face 

 .10, elevation of convex face .00 inch. 



Only two specimens of this pretty little form have come to our notice, 

 and these are respectively from widely separated localities, though evi- 

 dently from nearly the same horizon. One of the specimens, that from 

 Iowa, presents a group of teeth of which the larger and more perfect 

 one is that upon which the above specific description is based. The 

 other teeth of the group are all of smaller size, though p resenting a 

 remarkable persistency iu form, but they are not apparently in the rela- 

 tive order in which they occurred, being irregularly dispersed, having 

 the appearance of two contiguous rows of teeth crowded together, as 

 shown in the figure illustrating the convex aspect of the group ; other- 

 wise, if their occurrence iu the above specimen does indicate, their 

 relative natural position, it indicates a degree of irregularity in the 

 arrangement of the teeth such as has not been observed in other 

 Petalodont genera, of which series of teeth have been found in their 

 natural order. The crest of some of these teeth is interrupted by slight 

 vertical sulci, giving to the crown a delicate denticulate appearance, 

 which strongly recalls the appearance presented by Gtenopetalus serratm, 



