400 PALEONTOLOGY OF ILLINOIS. 



we have had opportunities for making direct comparison at the Museum of 

 Comparative Zoology. The apparent distinctive characters by which 

 the present form may be distinguished-froin the above mentioned species, 

 consists in the less deep serration of the crest, shallower concave face, 

 and relatively narrower base as compared with the lateral diameter of 

 the crown. 



Position and locality : Rare in the Chester limestone; Chester, Illi- 

 nois. 



Ctenopetalus medius, St. J. and W. 



pi. x a, rig. 26. 



The teeth herein referred to, and which we have, provisionally, recog- 

 nized as a distinct form, present the same general form and size as the 

 preceding species, G. Umatulus, but are recognized by the proportion- 

 ately greater lateral diameter of the crown as compared with the 

 entire hight of the tooth, which obtains in the majority of examples, 

 the narrower and obtusely pointed inferior extremity of the base, 

 the exceedingly minute and numerous serration of the somewhat 

 acuminate crest, each serrature being flanked, especially in the con- 

 vex face, by short, sharp crested carina, which regularly though 

 slightly diverge, but which are scarcely discernible in the absolutely 

 perfect condition, when the entire crown is invested with an exceed- 

 ingly delicate, polished enamel-like mantle. Iu the latter condition, 

 as also in shape, these teeth bear a marked resemblance to Pe.ta- 

 lodus linguifer, N. and W., a species described from the same for- 

 mation and locality ; but a careful comparison reveals other and 

 apparently permanent characteristics which seem to render its iden- 

 tity with that form extremely improbable. Thus, in the present teeth 

 the basal belt along the convex crown-face forms a simple, broad 

 fold in the inbeveled portion, sometimes showing quite distinctly along 

 the upper edge an extremely delicate imbrication, which only extends 

 through the broadly arched median region ; while, iu Petalodus linguifer, 

 the inferior or inbeveled area occupied by the coronal belt is distinctly 

 imbricated, even in minute examples, with three or more folds. The 

 iuner crown-face is moderately excavated and defined below by a well- 

 marked broadly downward-arched, imbricated coronal belt. 



We are aware of the variableness of the serration in G. serratus 

 Agassiz, but these variations are not apparently accompanied by other 

 features, such as distinguish the present teeth from Pelalodics linguifer. 

 On the other hand, the fine serration of the crest readily distinguish it 

 from G. Umatulus. 



