180 PALEONTOLOGY OF ILLINOIS. 



where the punctse are best seen, they show only obscure indications 

 of transverse disposition in more or less irregular parallel rows. 

 Length of a medium-size tooth at antero-lateral border about 9 mm ; 

 breadth across inner margin nearly 6 mm. 



The above description is based upon a few imperfect specimens, 

 which are supposed to represent the posterior teeth of the upper 

 jaw. In general outline and coronal contour they conform to the 

 characteristics of the genus. As already remarked, the punctse of 

 the triturating surface show an indistinct disposition in transverse 

 rows, in which respect the teeth differ roost from typical species of 

 the genus, though to not much greater extent than does the St. Louis 

 species, Xystrodus imitatus. It differs, however, from either the 

 St. Louis or Chester species, X. verus, in the relatively greater 

 breadth of the inner margin and consequent greater transverse ex- 

 pansion of the posterior alation. From the Upper Burlington 

 X. simplex it is distinguished by the greater lateral breadth and 

 convexity of the anterior coronal prominence. 



The collections have thus far afforded no recognizable examples 



identifiable with the broad posterior teeth of the mandible. 



Geological position and localities: Keokuk limestone, Hamilton and 



Warsaw, 111., Keokuk and Bentonsport, Iowa. 



XYSTRODUS IMITATUS, St. J. and W. 



PL VIII, Fig. 2. 



Maxillary posterior teeth small, triangular or sub-cuneate in out- 

 line, gently arched longitudinally. Antero-lateral border slightly 

 curved sigmoidally, forming above a sharp angle with the coronal 

 prominence, a narrow enamel belt folding over the upper edge, dis- 

 tinctly defined from the basal portion, which is nearly vertical or 

 projecting slightly outward, terminating in the attenuated inferior 

 edge, which apparently extends nearly the entire length of this side 

 of the tooth; inner margin sharply rounded from the anterior angle 

 with a slight concavity in its course to the slightly produced pos- 

 terior angle, and forming nearly a right-angle with the antero-lateral 

 border; postero-lateral border gradually diverging from the outer 

 extremity at an angle of about 30 with opposite border, very 

 gently curved, the upper edge enveloped in a narrow fold of the 

 coronal enamel, occupying about half the vertical height of the 

 border. Coronal surface slightly convex in the inner unworn por- 

 tion of the principal prominence, with a moderately steep slope into 

 the median depression, from which the surface more gently rises 



