3S6 PALAEONTOLOGY OF ILLINOIS. 



Poltkhizodus nanus, St. J. and W. 

 pi. xra, Fig. is. 



Teeth minute, stout, subelliptical in outline. Concave crown-face 

 broadly elliptical in outline, moderately excavated, basal margin mode- 

 rately prominent, broadly rounded, crest sharp, gently and regularly 

 arched ; convex face about half the hight of the opposite face, irregu- 

 larly lenticular in outline, basal margin forming an obtuse angle with 

 the rather narrow basal area, somewhat irregularly arched downward 

 and broadly curved to the acute lateral angles, and occupied by a nar- 

 row coronal fold, surface broadly arched laterally, gently convex verti- 

 cally and nearly at right angles to the basal area. Base nearly equal 

 to the lateral diameter of the crowD, in depth beneath the concave face 

 less than the elevation of the convex crown-face, thick, nearly vertical, 

 ■well-defined from the basal area beneath theconvex face, inferior surface 

 rounded, irregularly lobed with five, more or less, strong radicles. Late- 

 ral diameter of tooth .12 inch, greatest hight .07, elevation of convex 

 crown-face .04 inch. 



The above form is represented by a single example, which shows the 

 convex crown-face and basal aspect. The fact that this is the only repre- 

 sentative of the species, in a horizon abounding in ichthyic remains, is 

 not so surprising when we take into consideration its minute size, ren- 

 dering its discovery at all mere chance. We have not met with any 

 form with which the present is intimately allied. It is in every respect 

 dissimilar from P. Williamsi of the same horizon, being proportionately 

 more robust, and the base of greater lateral extent. In the compara- 

 tive strength and vertical diameter of the crown it seems to present 

 affinities with Dactylodus, but the short base is essentially characteristic 

 of Polyrtiizodus. 



Position and locality: Fish-bed horizon in the upper part of the 

 Keokuk limestone ; Bentonsport, Iowa. 



POLYBHIZODXJS PIASAENSJS, St. J. and W. 



PI. XHI, Fig. 12. 



Teeth of medium size, extremely elongated laterally. Crown mode- 

 rately strong, compressed and gently arched along the crest, which is 

 somewhat abruptly rounded to the acute lateral angles, nearly parallel 

 with the basal margin ; the convex face is nearly vertical, presenting 

 an irregular, flattened lenticular outline, slightly channeled, about half 

 the elevation of the opposite face, bordered by a prominent coronal 

 belt consisting of one or more narrow folds, forming an obtuse angle 



