46 PALEONTOLOGY OF ILLINOIS. 



nating in the prominent lateral angles. The root is large and 

 thick two-thirds the breadth, and nearly the height of the 

 crown. Its posterior face is flattened, the anterior slightly 

 arched both ways. It is divided nearly to the crown into five 

 finger-like rootlets, of which the two external ones are shortest 

 and most robust. 



These remarkable teeth differ widely from any hitherto described, in the long 

 fangs in which the root terminates; in this respect being more like the teeth of 

 mammals than fishes. The crown, however, is essentially that of Petalodus, 

 and there can be no doubt that they once belonged to a plagiostomous fish of 

 the same family as the typical Petalodi. 



Prof. McCoy has described some petalodont teeth, from the same horizon 

 with these, under the name of Polyrlvizodus, which also have roots divided into 

 several fangs ;and other similar species will be found represented in PI. in, and 

 described in the following pages. As will be shown hereafter, McCoy's Poly- 

 rMzodus has much in common with Petalodus. This affinity is also indicated 

 *e / A by the species before us, and D. lobatus, both being still more like fhe teeth on 

 which Owen founded his genus Petalodus ; and yet they have larger and more 

 ii^f v^lLV V ^''j^^/ distinct f angs. 



J The only specimen of D. princeps in the collection is, unfortunately, some- 



what worn and weathered, and its external surface has been in part removed. 

 It is, therefore, impossible to decide from that alone, whether in its normal state 

 it exhibited any decided departure from Petalodus. It now shows near the 

 summit of the crown a more porous structure than most species of that genus 

 in their perfect condition. As has been remarked, however, in the description 

 of Petalodus destructor, where the cutting edge of the crown is much worn, 

 and the polished enamel coating of either face is removed, nothing but a dotted 

 porous surface is seen ; the longer calciguous tubes which compose the edge 

 being removed or worn down, so that both there and below their ends alone are 

 visible. 



As has been suggested in the remarks on the genus Petalodus, we have 

 scarcely satisfactory data on which to divide the petalodont teeth into different 

 genera ; the typical species, the many rooted, the small rooted and the rootless 

 groups, all exhibiting among themselves nearly the same range of microscopic 

 structure; this variation, when existing in perfect teeth, following their forms 

 and consequently their uses. The broad-edged, grinding or crushing teeth of 

 each group exhibit, in their triturating surfaces, the homologues of the cutting 

 edges of the incisive forms ; the series of vertical enamel tubes being greatly 

 widened, and the tubes shortened. 



