206 PALEONTOLOGY OF ILLINOIS. 



prove them to be specifically identical. The individuals exhibit pre- 

 cisely the same variableness as regards the position of the truncate 

 angle of the articular border, though, as has been already remarked, 

 the facet is of greater extent ; indeed, in some instances occupying 

 nearly half the length of the border. 



Geological position and localities: Upper Burlington limestone, 

 chiefly from the main "fish-bed," a single example from the upper 

 fish-bed horizon; Buffington Creek, Pleasant Grove, Augusta, Iowa, 

 and Quincy, Illinois. 



PSAMMODUS TURGIDUS, St. J. and W. 



PI. XV. Fig. 4. 



A unique example of the maxillary tooth, derived from the fish- 

 bed of the Burlington-Keokuk " division beds," shows a still wider 

 divergence from Psammodus Springeri, and which we have provision- 

 ally recognized under the above specific designation. In outline the 

 tooth resembles the same form of P. Springeri and P. tumidus, its 

 relationship with the latter being most intimate. The inner border 

 is nearly equally divided into two long articular facets separated by 

 an angulation, the truncate facet being at the posterior angle, the 

 coronal belt very deep and well defined from the slightly channeled 

 basal portion. The inner coronal ridge is very prominent, occupy- 

 ing fully half the transverse diameter of the crown, broadly arched 

 in both directions and defined from the median depression by a 

 sharp impressed line; outer lateral border not known. The surface 

 shows a minute punctate structure through the dense enamel layer 

 enveloping the crown; along the posterior margin the surface pre- 

 sents a beautiful rugose appearance from the beaded longitudinal 

 plicae, which extend some distance from the margin, and the artic- 

 ular border as also the preserved portions of the anterior and pos- 

 terior margins are similarly ornamented with vertical rugosities. 

 The specimen measures between the angles of the inner border 

 15.5 mm, probably about the same as the greatest transverse diam- 

 eter at the posterior margin. 



A solitary and somewhat mutilated specimen, obtained by Mr. 

 Springer from the main fish-bed of the upper Burlington limestone, 

 presents exactly the same character of inner articular border that 

 distinguishes the above described tooth. But the coronal prominence 

 of the latter example, although very broad, is comparatively low and 

 merging into the shallow median depression, which was apparently 



