146 PALEONTOLOGY OF ILLINOIS. 



and I), undulatus, ample and variable in their bounds as they are, 

 do not make it clear that the present form is but a varietal phasis 

 of one or other of the last mentioned species. 



Geological position and localities: Keokuk limestone, near Warsaw 

 and Hamilton, 111. ; Keokuk, Iowa. 



DELTODUS CINCTULUS, St. J. and W. 



PI. IX, Fig. 6, 7. 



Teeth below medium size. Mandibular posterior tooth triangular 

 in outline, lateral diameter relatively broad compared to the length, 

 and somewhat strongly inrolled at the outer extremity. Antero- 

 lateral border nearly at right angles with a line connecting the 

 inner angles of the tooth. The upper half or more of its height 

 enveloped in the strong, inbeveled enamel fold, beneath which the 

 basal portion forms a narrow rim showing a coarse vascular struc- 

 ture ; inner margin with a slight sigmoidal curvature, broadly 

 arched round the base of the coronal ridge, with a moderate con- 

 cavity in passing the posterior depression into the posterior angle; 

 postero-lateral border gradually converging toward the outer margin, 

 occupied by a well-defined enamel fold extending nearly half the 

 depth, basal portion slightly channeled and terminating in a nar- 

 row rim inferiorly. Coronal ridge relatively broad, strongly arched 

 laterally, culminating a little anterior of the median line, with a 

 gradual slope into the posterior depression from which rises the rel- 

 atively narrow, slightly upraised alate lobe which terminates in the 

 sub-acute posterior angle; in mature or somewhat worn specimens 

 where the surface has been deprived of the original enamel investment, 

 the crown is undulated by sub-equal transverse folds and intervening 

 narrow sulci conforming to the bands of elongate punctse that mark 

 the order of increment of the tooth substance. Breadth of an ordi- 

 nary-sized specimen at the inner margin, 11.5 mm ; length of antero- 

 lateral border to point of inrollment, about 9 mm. 



Posterior tooth of the upper jaw trigonal in outline, moderately 

 arched in the longitudinal direction. Inner margin making a sig- 

 moidal curvature from the posterior sub-acute angle, broadly rounded 

 into the antero-lateral border with which it is more or less oblique ; 

 anterior articular edge unknown; postero-lateral border gradually 

 converging toward the outer extremity; enamel fold well defined 

 and separated from the basal portion by a narrow channel. Coronal 



