154 PALAEONTOLOGY OF ILLINOIS. 



round the base of the coronal ridge, with a slight concavity in the 

 region of the posterior depression, inferiorly inbeveled. Coronal ridge 

 relatively narrow, steeply rising from a faint depressed line near the 

 anterior edge, crest rounded, gradually descending into the posterior 

 wide depression, which is bordered by the moderately upraised 

 alation. Surface nearly smooth, with obscure transverse lines along 

 which the punctse exhibit elongated or otherwise enlarged openings, 

 rarely anything like undulations appearing. Length along crest of 

 coronal ridge to inrolled outer margin about 31 mm. ; length of 

 anterior articular border 18 mm. ; greatest breadth across inner 

 margin 21 mm. 



The collections from St. Louis horizons afford a solitary authentic 

 example only of the present form, the nearly perfect tooth above 

 described. This is quite distinct from the earlier-occurring species, 

 and offers striking resemblance in coronal contour with the smooth 

 individuals of Deltodus cingulatus, N. and W., of the Chester forma- 

 tion. It is, perhaps, distinguishable from the latter species by the 

 somewhat less produced posterior angle, less elevated coronal ridge, 

 and shorter articular edge of the antero-lateral border. A single 

 other imperfect example has been obtained by Mr. Van Home from 

 the same formation at Alton, Illinois, which represents the mandi- 

 bular posterior form possibly of the same species. The fragment 

 indicates a tooth of a relatively long, narrow outline with a rather 

 oblique inner margin in front, a broad gentle slope descending from 

 the crest of the coronal ridge to the deep, inbeveled coronal fold of 

 the articular border, the opposite flank abruptly descending into the 

 posterior depression; the alation, as also the outer margin of the 

 tooth, is broken away. 



Geological position and localities: St. Louis formation ; Pella, Iowa, 

 Alton, Illinois. 



DELTODUS POWELLII, St. J. and W. 



PI. X. Fig. 1. 



Teeth attain medium size. Mandibular terminal form subspatu- 

 late in outline, moderately arched longitudinally and rather strongly 

 inrolled at the outer margin. Antero-lateral border gently incurved, 

 enamel fold strongly inbeveled to the channeled basal rim ; postero- 

 lateral border gradually converges toward outer inrolled margin, 

 making an angle of about 23 with the anterior border, the rounded, 

 inbeveled enamel fold well denned by a narrow suleus from the 



