VERTEBRATES. 357 



the anterior margin is more vertical. In A. minutus the outline is elliptical, 

 and the imbricated folds more numerous and widely separted. 



Formation and locality : Burlington limestone; Burlington, Iowa. 



GENUS POLYRHIZODUS. 



PoLYRHIZODUS TRUNCATUS, N. aild W. 

 PI. iii, fig. 16, 16a. 



TEETH small, massive, sub-elliptical in outline ; crown gently 

 arched transversely on its upper and anterior face, which is 

 flattened and slightly excavated, and roughened by the oblique 

 sections of the calcigerous tubes; posterior face nearly straight, 

 laterally and vertically smooth or punctate ; this is bordered 

 below by four enamel folds which are slightly arched upward 

 at the ends ; root very small or obsolete. 



Although the specimens which we have of this tooth show little or no root, 

 there is scarce room for doubt that tubercular rootlets existed when it was in 

 a perfect condition ; the spongy texture of the root causing it to yield first in 

 the process ot decay or abrasion, when the tooth is detached from its support. 

 A comparison with the species we have named P. inflexus and P. porosus (vol. 

 ii, pp. 48, 49, PI. ii, figs. 8 and 9) will show that they should be placed in one 

 generic group with that now under consideration. 



From those species it is distinguished by its outline, lower and broader than 

 in P. inflexus, less low and broad than in P. porosus, and by a root more nearly 

 obsolete than in either. 



Formation and locality : Burlington limestone; Quincy, 111. 



POLYRHIZODUS LITTONI, N. and W. 



PI. iv, fig. 10. 100. 



TEETH of medium or large size, strong, oblong or elliptical in 

 outline, one and a quarter inch broad, half inch high; crown 

 low and depressed, superior margin broadly arched, subacute, 

 roughened by the extremities of the calcigerous tubes; ante- 

 rior face three lines high, lenticular in outline, with acuminate 



