116 PALAEONTOLOGY OF ILLINOIS. 



deemed sufficient to justify the separation of these individuals from 

 the more characteristic representatives of the species with which 

 they are associated. 



Compared with heretofore described species, the present one is 

 allied to that occurring in the Keokuk formation described by Messrs. 

 Newberry and Worthen, under the names Poecilodus ruyosus and P. 

 ornatus. l The species differ, however, in several marked peculiarities. 

 The closest resemblance between them is noted in connection with 

 the proportions and coronal contour of the mandibular posterior 

 teeth, the Upper Burlington form being mainly distinguishable by 

 the more delicate and close arrangement of the transverse imbrica- 

 tions. The median teeth of the mandibles of the two species differ 

 to a greater extent one from the other, the coronal prominence in 

 the present species possessing greater elevation and more pro- 

 nounced differentiation of the culminating ridge. In the posterior 

 teeth of the upper jaw the divergence in the contrasts between the 

 species is even more emphasized, the Upper Burlington examples 

 being destitute of the supplementary rugosities that surmount the 

 crest of the posterior lobe in the Keokuk species. 



It seems not, therefore, improbable that the examination of a 

 larger suite of specimens may reveal the existence of a contempora- 

 neous species during the Upper Burlington epoch, which the present 

 material rather suggests than demonstrates, much less affords the 

 necessary data for the satisfactory discrimination of its distinctive 

 characteristics. The illustrations convey a fair impression of the 

 species as determined from such remains as are at hand, while at 

 the same time they serve to show the variable appearance individual 

 specimens present, the limits of which are difficult to define. 



Geological position and localities : Upper Burlington limestone : Bur- 

 lington, Buffington creek, Louisa county, Pleasant Grove, Augusta, 

 Iowa ; Quincy, 111. 



CHITONODUS ANTIQUUS, St. J. and W. 



PI. VI, Fig. 2. 



Mandibular posterior tooth less than medium size, trapezoidal in 

 general outline, strongly inrolled and correspondingly arched longi- 

 tudinally. Antero-lateral border making nearly a right angle with 



NOTE. 'Illinois Geol, Surv., II, pp. 94, 95, PL VIII, ff. 13, 14. 



