376 PALEONTOLOGY OF ILLINOIS. 



TONAODTJS BELLICINCTTJS, St. J. and W. 

 PI. XI, Fig. 14-16, 25. 



Teeth of medium or small size, strong, in outline long-elliptical and 

 acutely rounded at the extremities. Concave crown-face but slightly 

 depressed, sometimes gently arched in the long direction of the tooth, 

 culminating along the convex margin in a sharp more or less regularly 

 arched crest, and bordered below by a wide, strongly marked basal 

 band, which is composed of three to six narrow and more or less regu- 

 lar, vertically striated imbricating folds, gently arched upward in the 

 middle and usually sharply curved round the extremities, iuferiorly 

 inbeveled and prominently defined from the root; conca\e face less 

 than half the bight of the opposite face, in outline arcuate and broadly 

 or gently arched laterally, generally very faintly channeled laterally, 

 the crest projecting beyond the basal line, and limited below by a more 

 or less well-defined basal baud, wbich presents along the greater extent 

 of the median portion only one or two distinct imbricating folds, but 

 which on nearing the extremities branch into several delicate secondary 

 folds, which apparently terminate in the crest, or, in part, from a con- 

 tinuous belt with that of the concave face. The coronal surfaces are 

 ornamented with vermiculose markings, immediately above the basal 

 baud faint vertical plicae, aud in the triturating surface along the crest, 

 where the exceedingly thin enamel-like layer has been ex-foliated, a 

 fine striato-punctatiou appears. The basal region forms a long, narrow 

 area, which, together with the face of the root, presents a broad, shallow 

 depression extending in the long diameter of the tooth; the root proper 

 is very short, autero-posteriorly compressed and laterally constricted, in 

 depth beneath the concave face nearly equal to or slightly exceeding 

 the bight of the convex crown-face, projecting outward and downward 

 in nearly the same plane as the convex crown-face, and iuferiorly flat- 

 tened. Lateral diameter of tooth .84 inch, transverse diameter of con- 

 cave crown face .24, hight of convex face .12, greatest length of root 

 .63, depth of inner face .09 inch. 



In the collection from the Chester limestone several varieties of teeth 

 occur, which are apparently intimately allied, though in some instances 

 it is exceedingly difficult to decide the nature of the differences by which 

 they are distinguished from one another. It is not difficult to separate 

 these teeth into three or four forms, all of which present a greater or less 

 individual variability; but in the extremes of some of these forms may 

 be observed such intimate resemblances with the teeth, referable to other 

 forms, as to cause one to hesitate in the determination of their identity. 

 Yet, however strongly we may suspect the validity of these groups, our 



