112 PALEONTOLOGY OF ILLINOIS. 



species being chiefly distinguishable by differences in proportions. 

 The absence of transverse rugae may be deemed of greater than 

 specific importance. The genus, however, presents in the ensemble 

 of its specific forms and the within bounds somewhat diverse char- 

 acteristics, features which indicate for it an important position at 

 the head of this particular group of the family, and from which 

 Cochliodus was probably an off-shoot at a later date. It embraces 

 besides the Burlington forms, species occurring in the Keokuk for- 

 mation described under the terms Cochliodus latus, Leidy, (Cochlio- 

 dus nobilis, N: and W.) Pcecilodus rugosus, N. and W., (including 

 P. ornatus, N. and W.), etc. We are not aware of the existence of 

 later representatives than those above enumerated from the Keokuk 

 formation and a single species from the St. Louis, the Chester 

 species belonging to typical Cochliodus. 



Were we in possession of sufficiently complete materials to con- 

 clusively demonstrate the relations of the species characterized by 

 the presence of the transverse imbrications or rugae, and those in 

 which the coronal surface is simply marked by more or less subor- 

 dinate revolving furrows and ridges, it might be found that these 

 constitute two distinct groups more or less well defined one from the 

 other. In that event Cochliodus latus would naturally fall into the 

 group represented by the primitive Burlington species Chitonodus 

 antiquus ; while on the other hand, the Keokuk "Pwcilodus rugosus" 

 would remain in association with the species here first described 

 under Chitonodus Springeri. 



CHITONODUS SPKINGBRI, St. J. and W. 



PL VI, Fig. 3-15. 



Mandibular posterior teeth trapezoidal in outline, strongly in- 

 rolled. Antero-lateral border with a slightly oblique course outward 

 and forward, of moderate height, coronal enamel forming a narrow 

 band abruptly folded over the edge and inbeveled to the channeled, 

 closely pitted basal portion; postero-lateral border converging to- 

 wards point of inrollment at an angle of about 50 with the 

 opposite side, coronal enamel forming a well-marked rounded fold 

 along the upper edge of the channeled basal portion ; inner margin 

 making a right angle with the antero-lateral border, gradually pro- 

 duced backward from the obtuse anterior angle and sharply curved 

 round the base of the coronal prominence, with a corresponding 

 deep sinuation in passing the posterior depression into the subacute 

 posterior extremity, ^usually inbeveled. Superficial characters pro- 



