140 PALEONTOLOGY OF ILLINOIS. 



posterior or principal ridge occupying half the lateral diameter of 

 the tooth, rather strongly convex transversely, posterior slope mod- 

 erate and uniform to the postero-lateral border, opposite slope steeply 

 descending with slight concavity into the narrow median depression 

 which corresponds in the extreme obliquity to the course of the an- 

 terior articular border; anterior ridge culminating in a sharply 

 rounded crest, toward the outer margin flanked by the nearly verti- 

 cal, concave slope of the median depression, the border opposite 

 declivity more gently descending to the antero-lateral border along 

 which it is slightly depressed, and marked by faint longitudinal un- 

 dulations, or sometimes interrupted plicae which obliquely descend 

 from near the crest toward the oblique anterior portion of the inner 

 margin. Length of a specimen of ordinary size along the antero- 

 lateral border about 6 mm. ; transverse diameter at the inner margin 

 17 mm. 



Triturating surface finely and closely punctate in both forms men- 

 tioned above, presenting the usual variations in accordance with' the 

 degree of abrasion to which the surface has been subjected. In 

 crossing the narrow ridges the punctse often show marked lateral 

 elongation and disposition in parallel rows corresponding in course 

 to the formative margin, besides the oblique and longitudinal folds 

 already mentioned occurring in the anterior slope of the maxillary 

 teeth ; and in excessively worn surfaces the medulary layer is entirely 

 removed, exposing to view the dense homogenous inferior layer of 

 the dental structure. 



Only a few specimens representing the above described forms have 

 been obtained, associated in the same stratum at LaSalle ; examples 

 of probably specifically identical forms have come to our notice from 

 other localities in Illinois and Kansas. The species offers characters 

 readily distinguishing it from allied earlier appearing forms; the 

 longitudinally ridged median depression and relatively narrow alate 

 expansion of the mandibular terminal teeth, and the relative pro- 

 portions of the coronal prominences and the peculiar undulations of 

 the anterior slope in the maxillary teeth constitute the salient fea- 

 tures contrasting the species with P. Cestriensis. 



Geological position and localities : Upper Coal Measures ; LaSalle, 

 111., and Kansas. A single specimen has been obtained from the 

 dark limestone twenty feet above Coal No. 6, near Belleville, and 

 Mr. Butters has discovered the same remains in the roof shales of 

 Coal No. 5 at Carlinville, this State. 



