92 University of Michigan 



PLATE XVI 



Cyrtotoma. 



Figure 8. C. mexicanmn salleanum. Radula. 

 Figure 9. C. m. salleanum. Aperture. 



Figures 10, 11, 12. C. m. mexicanum. Apparent stages in develop- 

 ment of aperture ; drawn from different specimens. 



