362 PALAEONTOLOGY OF ILLINOIS. 



the same species. Among themselves the various forms are linked 

 together by the unmistakable gradation of characters, "while at the 

 same time they are not easily confounded with others which are regarded 

 as specifically distinct. Of the latter, the Keokuk and Burlington for- 

 mations have afforded many beautiful examples, and which exhibit 

 precisely the same range of individual variation, though associated with 

 other features of presumably distinctive importance. The latter forms 

 were originally indicated under the name Helodus angulatus in the 

 second volume of the present report. 



Indubitable examples of the form under consideration have been 

 obtained only from the St. Louis limestone, at a single locality in the 

 northern extension of the formation in Central Iowa. However, our 

 collections contain a few examples of teeth from the Warsaw beds, 

 which are difficult to distinguish from the present form, since they present 

 precisely the same variations as have been observed in connection with 

 the St. Louis specimens. Most of these teeth are so worn that a satisfac- 

 tory comparison is out of the question ; but a single specimen obtained 

 by Mr. Van Horne in the Warsaw beds at Barrett's Station, Missouri, 

 is deserving of particular notice : The tooth is of average size, and is 

 distinguished by its linear or very slightly curved convex border, nearly 

 uniform and prominent crest, relative great depth of the convex face, 

 the lower half of which is occupied by a broad coronal belt composed 

 of five or six strong, regular imbricating folds, which are seldom 

 branched or implanted, concave face rising abruptly from the narrow 

 marginal border, which is apparently composed of two folds, and present- 

 ing a relatively narrow, deeply channeled outline quite in contrast to 

 that observed in the numerous representatives from the St. Louis beds. 

 The base formed a strong plate projecting downward in a plane closely 

 corresponding to that of the convex crown-face, from which it is defined 

 by the produced and sharply iubeveled marginal border ; the convex 

 crown-face is coarsely striato-punctate in the triturating surface along 

 the crest, the opposite face is smooth and polished. From the same 

 horizon, in the exposures above the mouth of Piasa creek, Mr. Van 

 Hokne has obtained other specimens, but which, in the abraded con- 

 dition, afford at best unsatisfactory comparison with the allied if not 

 identical form from the superimposed beds. 



The Chester limestone, also, have afforded a few teeth, which present 

 the same general features, but which, unfortunately, are not sufficiently 

 well-preserved to trace more intimate relationship than in the case of 

 the Warsaw form. They are, however, apparently distinguished by the 

 fewer, stronger, and more regular coronal folds, perhaps more rectan- 

 gular outline, stronger base, and perceptibly finer puuctation. But in 

 view of the variableness of the surface structure under the varying 



