VERTEBRATES. 303 



conditions of the preservation of the crown, little dependence can be 

 placed on the latter feature. 



Of the last mentioned forms, which we have provisionally indicated 

 by the designations G. Yarsouviensis and C. Cliesterensis, we have intro- 

 duced illustrations of a few of the better preserved specimens, hoping 

 by this means to call attention more particularly to their consideration 

 when the organization of more complete materials shall enable a satis- 

 factory comparison with the form described above, and thereby settle 

 their relationship with one another. 



Position and locality : Common in the calcareous shales of the St. 

 Louis formation ; Pella, Iowa. The locality of the Warsaw form is 

 indicated above ; the Chester form occurs at Chester, Illinois. 



Ge^us LISGODTTS, St. J. and W. 



Teeth laterally abbreviated, strong. Crown thick, or moderately 

 compressed, more or less acuminate, sharp-crested, aud sometimes 

 obscurely serrated, basal margins well defined from the base, descend- 

 ing lowest, or most produced, in the concave face, which is more or less 

 strongly arched laterally, the convex face lowest, forming a more or 

 less vertical plane area, coronal belt encircling the crown, and composed 

 of one or more imbricating folds. Base nearly vertical to the crown, 

 rectangular in outline, thick, less than the lateral diameter of the crown, 

 in depth equal to or exceeding the elevation of the crown, inferior sur- 

 face well-defined from either face above, aud generally slightly beveled 

 from the concave to the opposite border. Coronal surfaces invested 

 with a polished enamel-like layer, the worn crest striato-punctate. 



In regard to the affinities of the forms herein included to the species 

 with which they are associated, we have sought in vain to discover 

 features which would more than vaguely suggest their intimate relation 

 to, much less their specific identity with, other generic forms. No gra- 

 dation into the elongated forms of Chomatodus is observed, such as 

 would hardly fail to be represented, did they exist, in the splendid col- 

 lections which have been so generously made available to us; while 

 the mutual identity of characters of the several forms included in the 

 present group would seem to be sufficient evidence of their generic dis- 

 tinctness. Tet one cannot fail to note the affinities or resemblances 

 which these teeth bear to typical Chomatodus, as represented by 0. 

 pusillus, X. and W., or the representative of that species in the Upper 

 Burlington fish-bed ; and this gives rise to the suspicion that the acqui- 

 sition of a series of the latter teeth in their relative natural position 

 might prove these to belong to the dentition of one and the same form — 

 an arrangement something like the triturating plates of the Myliodonts. 



