PLATE IX.— Continued. 



Aparchites unicornis, var. (page 50). 



Figure 11. A valve of this species having the ends less equal than in the 

 typical form, x 18. 



Strepula QUADRiLiRATA, (page 54). 



Figure 12. A perfect right valve of this species, x 18, frora the Hudson River 

 group at Clarksville, Ohio. 



Strepula quadrilirata, var. simplex (page 55). 

 Figure 13. View of the only specimen of this variety seen, x 18. 



Strepula lunatifera, (page 56). 



Figure 14. Large right valve, x 18, having the crescentic space separated from 



the marginal ridge. 

 Figure 14a. Posterior end view of the same. 

 Figure 14&. Longitudinal section of the same, showing the convexity of the 



valve and the elevation of th'e ridges. 



