374 PAL JE ONTOLOGY OF ILLINOIS. 



some instances diverging from the median line in their upward Ci ;use 

 until lost in the papillose surface occupying the upper space, as also the 

 greater portion of the convex face, and the striato-punctate belt imme- 

 diately along the crest; also, in well-preserved specimens, the coronal 

 folds are more or less distinctly and finely striated vertically. The basal 

 area nearly corresponds in outline to the concave crown-face, in contour 

 moderately convex, rarely plane, defined from the basal line of the con- 

 vex crown-face by a faint sulcation. gradually curved downward and 

 merging into the short, wedge-shaped, laterally constricted and verti- 

 cally sulcated root, the inner face of which forms an abrupt declivity 

 projecting slightly outward, laterally channeled, and nearly parallel 

 with the concave coronal margin from which it is more or less sharply 

 defined by the produced border of the coronal folds ; the inferior surface 

 in the majority of the specimens is reduced to an exceedingly narrow 

 area, giving to the root the appearance of terminating in a sharp edge ; 

 but in well-preserved examples it forms a well-marked though very 

 shallow and relatively narrow appendage, the flattened inferior surface 

 of which agrees with the horizontal plane of the crown, and at the same 

 time it is more abruptly defined on the one side from the broad basal 

 region. A tooth of large size measures .48 inch in greatest diameter of 

 crown, hight of concave face nearly .30, elevation of convex face .OS, 

 greatest diameter of root .30, depth beneath the concave coronal border 

 .10; a minute, oval-shaped tooth is .20 inch in lateral and .11 in trans- 

 verse diameter, hight of convex crown face .05, or about equal to the 

 depth of the root. 



Although hardly any two individuals of this elegant little form pre- 

 sent precisely the same outline, in which respect it is very variable, yet 

 all are easily recognizable by the peculiar coronal sculpturing as well 

 as by the general contour features common to all. The form is appa- 

 rently restricted to the St. Louis division, and the examples at present 

 known, which were derived from localities more or less distant from one 

 another, present a persistency of characters which afford the best evi- 

 dence of their specific identity. Its most intimate ally is found in the 

 upper (?) ichthyic horizon of the next succeeding or Chester formation, 

 and so close is their relationship, that it is only by comparing a suite of 

 each form that they may be satisfactorily distinguished one from the 

 other. However slight these distinctive features may appear in the 

 comparison of solitary examples from either horizon, collectively, each 

 presents a combination of features which at once arrests the attention 

 and leads to the conviction of their specific distinctness. These differ- 

 ences are noticed more at length in the description of T. polymorphus. 



In regard to the generic relations of the present form, it would appear 

 that, in the outline of the crown, it approximates Antliodus ; but by 



