106 ROYAL SOCIETY OF CANADA 
HYOLITHES CAUDATUS, OPERCULE, Horizon and locality as above 
(Fig. 5a.) 
The two diameters equal, 14 mm. This is the usual size. This 
opercule differs from those of the preceding species in the broad thick 
chevron, widening rapidly toward the margin, which however it does 
not reach. In place of the crescentic ridge behind the apex of the 
cone this species has a V-shaped ridge, dividing off at the back a trian- 
gular space, with a slight ridge in the middle. One vascular line was 
observed at the side of the cone. The margin of the opercule has a 
faint marginal furrow. 

