434 



NORTH AMERICAN INDEX FOSSILS. 



193. P. ponderosus Hall. (Fig. 568.) 

 Shell gibbous, large. 

 Onondaga : New York, Ohio, Ontario. 



Devonic. 



Plethomytilus ponderosus X^« (Pal. N. Y. V. ) 



194. P. oviformis Conrad. (Fig. 567, d } e.) Devonic. 



Differs from P. ponderosa in its proportionately longer hinge 

 line, smaller size, and less gibbosity. 



Hamilton : New York, Pennsylvania. Lower Devonic of 

 Nevada. 



LX. Clionychia Ulrich. 



Resembles Ambonychia but differs in its smaller and but little 

 incurved beaks, in the possession of concentric striae alone, and in 



Fig. 569. #, Clionychia lamellosa, left valve ; b, C. undata, left valve. (After 

 Ulrich, Minn. Surv.) 



the thickening of the hinge margins anteriorly, producing in inter- 

 nal molds a well-marked impression in this part. 



