72 PALAEONTOLOGY OF ILLINOIS. 



Petrodus acftus, N. and W. 



PI. IV, Pig. IT. 



There is in the collection a single specimen of Petrodus that 

 is derived from a different locality from the others, and which 

 is so unlike them as to justify its separation from them. It is 

 small in size, elliptical in outline, and the carinoe of the surface 

 do not radiate from a central point, but the line of the longest 

 diameter is marked by a prominent, acute crest, from which 

 4—5 pairs of acute, simple ridges are given off pinnately run- 

 ning down to the base. All the carinse are, as compared with 

 those of P. occidentalis, much sharper. 



Figure 17, a small specimen seen from above, twice natural size. 

 Formation and locality* Coal Measures, Pike county, Illinois* 



Genus CTENOPTYCHIUS, Ag. 



Gen. Char. — Teeth small, highly polished, strongly com- 

 pressed, rounded or obtusely pointed, edge divided into several 

 strong denticulations; base of crown with a few imbricating 

 folds of ganoine; bony root oblong, flattened in the same 

 direction as the crown. 



It is very apparent that the species which have been referred to Ctenopty- 

 cliias require separation; as, for example, Ct. wpicalis, Ag., can hardly belong 

 to the same genus with Ct. serratus of Owen ; but we have not now the mate- 

 rial for a discussion of the question of which should retain the name of Cten- 

 optychius. 



Ctenoptychius semicircularis, N. and W. 



PI. IV, Figs. 18, 18 a, 18,6. 



Teeth of median size ; crown very thin, much compressed, 

 and laterally curved, when viewed from above in outline form- 

 ing a full semi-circle; anterior and posterior surfaces smooth 



