VERTEBRATES. 51 



Polyrhizodus ponticulus, N. and W. 



PI. Ill, Fig. 11 and 11 a. 



Teeth broad, low and thin, in outline oblong or linear; pos- 

 terior surface of crown two-thirds the entire height, concave 

 vertically, nearly straight laterally, smooth and polished ; 

 imbricating folds forming a na,rrow, low basal band; anterior 

 face nearly horizontal, giving a flat or slightly arched summit 

 to the tooth, not quite half as broad as the height of the poste- 

 rior face ; basal ridge a sharp angle without distinct folds; base 

 forming half the height of the anterior surface, concave verti- 

 cally, gently arched laterally, smooth and polished; root verti- 

 cal, nearly as broad as the tooth, and composing one-third of 

 its height, flat, thin and divided into several sub-equal quad- 

 rangular rootlets. Height of tooth 4 lines; breadth of a 

 portion only, 15 lines. 



This tooth should evidently be classed in the same genus with McCoy's 

 Polyrhizodus magnus (Brit. Palseoz. Foss. p. 641, PI. 3 7c, fig. 6, 7, 8), but cer- 

 tainly not with his P. pusillus, ( Op. Cit. p. 642, PI. 3, k, fig. 2.) 



From the former it differs, however, in being lighter throughout, the more 

 angular crown, shorter, fewer, and more angular roots. Its relations to the 

 preceding species, P. dentatus, are much closer, that species differing from this 

 only in being higher, the crown more acute above, the radicles more numerous 

 and much thicker. In both these species the summit of the crown forms nearly 

 a straight line throughout, the basal ridge being curved up to it at either end. 



Figures 11, 11 a, represent the anterior aspect and profile, natural size. As 

 will be noticed, this face of the tooth bears a striking resemblance to a many- 

 arched bridge, a resemblance which suggested the name given it. 



Formation and locality: Chester limestone, Chester, Illinois. 



Genus CHOMATODUS, Ag. 

 Chomatodus gracillimus, N. and W. 



PI. Ill, Fig. 12,. 12 a. 



Teeth transversely elongated, linear in outline; posterior 

 crown face half the entire height with rounded ends, its upper 



