222 PALEONTOLOGY OF ILLINOIS. 



curved; the outer border broadly arched, the basal portion project- 

 ing in a prominent, thick rim beyond the coronal limits, slightly 

 produced posteriorly, and inbeveled to the inferior surface ; the inner 

 border presents precisely the conformation observed in typical exam- 

 ples, except that the angulation is placed forwards a little less than 

 one-fifth the distance from the anterior angle, distinctly denning 

 the anterior articular facet from the long posterior articular surface. 

 The coronal surface is longitudinally moderately arched, and trans- 

 versely depressed, limits of the crown well denned at the sides, the 

 inner ridge presenting the abrupt face noticed in the above men- 

 tioned example from Pella ; the outer ridge also rises into a low 

 accumination nearest the anterior angle. The worn surface exposes 

 the usual minute punctate structure, but in the more depressed 

 portions which have escaped abrasion, the transverse rugae are dis- 

 cernible, the same as noted in one of the Alton specimens of a very 

 elongate tooth. 



The present species differs equally from that noticed from the 

 Upper Burlington limestone, and Psammodus canaliculatus, McCoy, 

 of the Irish Mountain limestone : Relatively shorter than the latter, 

 from the former it may be distinguished by the less prominent and 

 more rounded contour of the coronal ridge along the outer border. 



Geological position and localities: St. Louis formation ; Alton, 111. ; 

 Pella, Iowa. 



PSAMMODUS ANGULARIS, N. and W. 



PI. XIX, Fig. 1, 2. 



F><ai>imi>flits angular is, NEWBEKRT and WOBTHEN, 1866, 111. Geol. Surv. II, p. 107, PI. XI* 

 f. 2, 2a, 21>. 



Psammodus porosus? Agass., ib., p. 107, PL XI, f. 1, la, lb. 



The above designated species was described from a nearly perfect 

 example belonging to the form provisionally referred to the mandible. 

 In order to facilitate comparison with the forms described in pre- 

 ceding pages, we venture to supplement the brief description above 

 cited by a somewhat detailed notice of the species, based upon the 

 original example and the few additional specimens subsequently dis- 

 covered in the same horizon whence the type was derived. 



The supposed mandibular teeth attain medium size, showing a 

 trapezoidal outline. The coronal region is moderately and regularly 

 arched from behind forwards, gently depressed transversely, with, 

 however, a fullness in the inner half or more of the area, which 



