78 TREN'JON PERIOD. 



Width of the umbiHcus about equal to the transverse diameter of the aper- 

 tiu-e, which is a httle gi-eater than one-third of the full transverse diameter 

 of the shell. 



Transverse diameter of the shell, nine millimeters; height of same, two 

 and a half millimeters. 



This shell differs shghtly from the species, as described and fig-ui-ed by 

 Mr. Meek {loc. cit.), bvit it is probable that it is not specifically difi"erent. 



Position and locality. — Silurian strata, probably of the Trenton period; 

 Ewell's Spring, Arizona (lower horizon). 



