VERTEBEATES. 49 



Genus POLYRHIZODUS, McCoy. 

 Polyrhizodus porosus, N. and W. 



PI. Ill, Figs. 9, 9a. 



Teeth thick and massive, twice or three times as broad as 

 high; posterior face of crown comprising nearly half the height 

 of the tooth, long-elliptical in outline, with rounded extremities, 

 strongly inclined backward toward the summit, very slightly 

 concave both vertically and laterally, smooth in perfect speci- 

 mens, marked with fine vertical striations in those that are 

 weathered — in such, a narrow band of pores are visible along 

 its lower margin; coronal imbricating folds forming a broad, 

 flattened band, nearly as high as the crown face, laterally 

 straight except near either end, where it is suddenly curved 

 upward, folds three, of equal height, with two very fine lines 

 below; anterior face nearly horizontal, forming a triturating 

 surface of the same form and area as the posterior crown face, 

 slightly concave and lower before. In unworn specimens this 

 is covered with a coating of polished enamel, except at the poste- 

 rior edge, where it is porous; when much worn the entire surface 

 is distinctly porous, as in Psammodus; base as high as anterior 

 crown face, to which it is inclined below, smooth throughout, 

 convex vertically and laterally; root nearly as wide as the 

 tooth, directed forward and downward, and reaching but little 

 below the line of the posterior basal folds, divided into several 

 distinct radicles. 



The form of the crown of these teeth is similar to that of Polyrhizodus mag- 

 nus, McCoy (loc. cit.), hut they are smaller, have shorter rootlets, and a some- 

 what different profile; Prof. McCoy's species having the lower margin of the 

 posterior crown face inclined backward, while in our species it is inclined 

 forward, etc. 



Figures 9 and 9 a represent the posterior aspect and profile of the largest 

 specimen in the collection, of the natural size. 



Formation and locality: Burlington limestone, Burlington, Iowa, and Quincy, 

 Illinois. 



—7 



