68 PALEONTOLOGY OF ILLINOIS. 



channeled and undulated as in the opposite border; inner margin 

 broadly rounded, base excavated as in the previously described form. 

 Crown very slightly arched transversely, well-preserved specimens 

 showing two obscure longitudinal prominences, one near and rising 

 rather abruptly from the posterior border, the other a little anterior 

 of the middle ; a third, and perhaps sometimes quite as well marked 

 ridge, occurs just back of the anterior border. Coronal surface finely 

 and closely punctate. A medium-sized tooth measures across the 

 inner margin 29 m m ; ditto, outer margin 16 m m ; length along 

 anterior border 21 mm; ditto, posterior border 32 m m ; depth of 

 tooth at middle of anterior border 3mm; ditto, inner margin about 

 5mm. 



A very small specimen which is provisionally identified with the 

 maxillary form of the present species, shows a proportionately shorter 

 tooth of a more rhombic outline, the outer margin, also anterior 

 and posterior borders, distinctly crenulated, the former strongly so, 

 and further distinguished by the gradual convergence of the lateral 

 borders, greater and more regular transverse convexity of the coronal 

 region, as contrasted with the typical specimens. These features of 

 coronal contour are also intimately shared by a more elongate ma- 

 ture tooth, but which, though imperfect along the anterior border, 

 and considerably worn towards the outer extremity, evidently closely 

 agrees in outline with the typical examples. 



Compared with other American Carboniferous forms of Psephodus, 

 the present offers characteristics readily distinguishing them specifi- 

 cally. From P. crenulatus (N. and W.) of the Chester formation, 

 the species differ in the uniformly larger size of the teeth, repre- 

 senting the dentition of different parts or opposite jaws. The large 

 median teeth, probably belonging to the lower jaw, differ in being 

 less massive, possessing fewer and less distinct undulations in either 

 border supposed to have relation to contiguous series of smaller 

 teeth, and the .less strongly marked crenulations of the narrow 

 coronal fold. The depressed trapezoidal teeth included in the second 

 form described under the above specific caption, and which are sup- 

 posed to have belonged to the upper jaw, differ from the corres- 

 ponding teeth of the Chester species in essentially the same respects 

 above cited, with perhaps less marked obliquity of inner and outer 

 margins and less strong spiral inrollment. Examples of both of 

 the above forms of the Chester species show the entire margins and 

 borders of the teeth to have been crenulated ; besides they are more 

 or less deeply notched in one or other border, showing the manner 



