396 PALAEONTOLOGY OF ILLINOIS. 



the collection of Prof. Worthen contains several more or less frag- 

 mentary examples, besides the nearly perfect tooth figured. These 

 fragments exhibit a uniformity of characters which has led provision- 

 ally to their recognition as a distinct form. From Petalodus Alleghani- 

 ensis, Leidy, with which they are associated, they are distinguished by 

 the regularly arched crest, the comparatively horizontal direction of 

 the basal border of the concave face, relatively low convex face, and 

 apparently short, broadly rounded base, which is produced downward 

 in nearly the same vertical plane as the convex crown-face, compara- 

 tively slight antero-posterior diameter, and about two-thirds the lateral 

 diameter of the crown. The concave crown-fiice presents an elliptical 

 outline, broadly rounded at the extremities, and nearly as high again 

 as the opposite face, which latter is irregularly lenticular in outline. 

 The basal belt in the concave face is composed of three or four wide, 

 regular imbrications, occupying more than a third of the entire eleva- 

 tion, about half their breadth produced beyond the plane of the base ; 

 in the convex face the coronal belt is composed of four or five narrower 

 folds, about half the width of that in the opposite face, and strongly 

 inbeveled to the basal region ; the crest is compressed, with the usual 

 vertical striation produced by the abrasion of the enamel-like layer and 

 exposure of the tubular structure. Lateral diameter of crown of a full- 

 sized specimen .8S inch, hight of concave face .35, hight of convex face 

 about .20 inch. 



It will be observed that the teeth described present striking features 

 in contrast with the large teeth originally described by Dr. Leidy under 

 the name Petalodus Alleghaniensis, the originals of which he kindly sub- 

 mitted to us for examination, and to which we are satisfied belong the 

 teeth subsequently described in the second volume of the present report 

 under the name P. destructor, N. and W. The teeth under considera- 

 tion possibly hold the same relation to the above mentioned form as 

 does P. Hastingsii, Owen, to P. acuminatus, Agass., of the British Car- 

 boniferous limestone. Prof. McCoy has suggested the probable specific 

 identity of the latter forms; but we have never seen immature or small 

 individuals of P. Alleghaniensis that might be confounded with the 

 present teeth, although such may yet be discovered. 



Position and locality : In the limestone above coal No. 8, Upper Coal 

 Measures; near Springfield, Illinois. 



Genus CTE^OPETALUS, Agassiz. 

 Ctenopetalus yinostjs, St. J. and W. 



PI. in, Fig 13 



Under the designation Ctenopetalus, Prof. Agassiz recognized the 

 generic distinctness of a Petalodont form of teeth, which had previously 



